arm64: zynqmp: zcu106: fix SPL MMC booting

arm64: zynqmp: zcu106: fix SPL MMC booting

The U-Boot SPL generated with the current zcu106 defconfig cannot boot
from MMC:

  [...]
  U-Boot SPL 2018.01 (Feb 21 2018 - 17:47:14)
  EL Level:  EL3
  Trying to boot from MMC1
  sdhci_transfer_data: Error detected in status(0x408020)!
  spl_load_image_fat_os: error reading image u-boot.bin, err - -2
  spl_load_image_fat: error reading image u-boot.img, err - -6
  SPL: failed to boot from all boot devices
  ### ERROR ### Please RESET the board ###

Fix by lowering the rpll value. The new value for the RPLL_CTRL
register comes from the current psu_init_gpl.c from the HDF file at
https://github.com/xilinx/hdf-examples/tree/01ad8ea5fd1989abf4ea5a072d019a16cb2bc546/zcu106-zynqmp
(generated by Vivado v2017.4).

RPLL and sdio1_ref clocks before and after this change:

 - Old values: RPLL 1.36 GHz, sdio1_ref 272 MHz
 - New values: RPLL 1.16 GHz, sdio1_ref 233 MHz

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Cc: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>