hw/riscv: microchip_pfsoc: Map EMMC/SD mux register

hw/riscv: microchip_pfsoc: Map EMMC/SD mux register

Since HSS commit c20a89f8dcac, the Icicle Kit reference design has
been updated to use a register mapped at 0x4f000000 instead of a
GPIO to control whether eMMC or SD card is to be used. With this
support the same HSS image can be used for both eMMC and SD card
boot flow, while previously two different board configurations were
used. This is undocumented but one can take a look at the HSS code
HSS_MMCInit() in services/mmc/mmc_api.c.

With this commit, HSS image built from 2020.12 release boots again.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20210322075248.136255-1-bmeng.cn@gmail.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>