riscv: sifive: fu540: Enable SiFive SPI and MMC SPI drivers

This patch enables SiFive SPI and MMC SPI drivers for the
SiFive Unleashed board.

Signed-off-by: Bhargav Shah <bhargavshah1988@gmail.com>
Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>