arm: ls1021a: drop redundant board_mmc_init()

The board_mmc_init() defined in board files is actually
doing same thing with the cpu_mmc_init() defined in
arch/arm/cpu/armv7/ls102xa/cpu.c. So drop it.

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>