mmc: sdhci-of-dwcmshc: Add HPE GSC eMMC support

mmc: sdhci-of-dwcmshc: Add HPE GSC eMMC support

Add support for the eMMC controller integrated in the HPE GSC (ARM64
Cortex-A53) BMC SoC under the new 'hpe,gsc-dwcmshc' compatible
string.

The HPE GSC eMMC controller is based on the DesignWare Cores MSHC IP
but requires several platform-specific adjustments:

Clock mux (dwcmshc_hpe_set_clock):
  The GSC SoC wires SDHCI_CLOCK_CONTROL.freq_sel directly to a clock
  mux rather than a divider.  Forcing freq_sel = 1 when the requested
  clock is 200 MHz (HS200) selects the correct high-speed clock source.
  Using the generic sdhci_set_clock() would otherwise leave the mux on
  the wrong source after tuning.

Auto-tuning / vendor config (dwcmshc_hpe_vendor_specific):
  Disables the command-conflict check (DWCMSHC_HOST_CTRL3 BIT(0)) and
  programs the ATCTRL register using existing AT_CTRL_* macros:
    AT_CTRL_AT_EN           auto-tuning circuit enable
    AT_CTRL_SWIN_TH_EN      sampling window threshold enable
    AT_CTRL_TUNE_CLK_STOP_EN tune-clock-stop enable
    PRE_CHANGE_DLY  = 3     pre-change delay
    POST_CHANGE_DLY = 3     post-change delay
    SWIN_TH_VAL    = 2      sampling window threshold
  This combination is required for reliable HS200 signal integrity on
  the GSC PCB trace topology.

eMMC mode (dwcmshc_hpe_set_emmc):
  Helper that sets DWCMSHC_CARD_IS_EMMC unconditionally.  Called from
  both the reset and UHS-signaling paths.

Reset (dwcmshc_hpe_reset):
  Calls dwcmshc_reset(), re-applies the vendor config above via
  dwcmshc_hpe_vendor_specific(), and then calls dwcmshc_hpe_set_emmc().
  The GSC controller clears the CARD_IS_EMMC bit on every reset;
  leaving it clear causes card-detect mis-identification on an
  eMMC-only slot.

UHS signaling (dwcmshc_hpe_set_uhs_signaling):
  Wraps dwcmshc_set_uhs_signaling() and calls dwcmshc_hpe_set_emmc()
  to ensure CARD_IS_EMMC is set for all timing modes, not just HS400.

Init (dwcmshc_hpe_gsc_init):
  Obtains the SoC register block and MSHCCS offset via the
  'hpe,gxp-sysreg' syscon phandle argument and sets SCGSyncDis
  (BIT(18)) to allow the HS200 RX delay lines to settle while the
  card clock is stopped during auto-tuning.  Enables SDHCI v4 mode.

Quirks:
  SDHCI_QUIRK_CAP_CLOCK_BASE_BROKEN:  base clock not advertised in
    capabilities; must be obtained from the DTS 'clocks' property.
  SDHCI_QUIRK2_PRESET_VALUE_BROKEN:  preset-value registers are not
    populated in the GSC ROM.

All HPE-specific code is isolated to the new hpe_gsc_init / hpe_ops /
hpe_gsc_pdata symbols.  No existing platform (Rockchip, T-Head, sg2042,
etc.) is affected.

Signed-off-by: Nick Hawkins <nick.hawkins@hpe.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>