XR819 seems to need a delay after its reset line to be deasserted,
otherwise it may not respond MMC commands correctly, and fail to
initialize.

Add a 200ms delay in the mmc-pwrseq.
Signed-off-by: Icenowy Zheng <icenowy@aosc.xyz>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>