riscv: fix nommu_k210_sdcard_defconfig

commit 762e52f7 upstream.

Instead of an arbitrary delay, use the "rootwait" kernel option to wait
for the mmc root device to be ready.

Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Fixes: 7e09fd39 ("riscv: Add Canaan Kendryte K210 SD card defconfig")
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>