doc: sifive-fu540: Update README for SiFive SPI and MMC SPI drivers

This patch removes SiFive SPI driver and MMC SPI drive from the TODO
list in SiFive FU540 README.

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>