MIPS: eyeq5_defconfig: add cadence MMC/SDHCI driver

Enable MMC support on eyeQ5 platform so it can be used as the root
partition.

Signed-off-by: Benoît Monin <benoit.monin@bootlin.com>
Acked-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>