power: regulator: rk8xx: Add 500us delay after LDO regulator is enabled

A quick power cycle of a LDO regulator during dw-mmc signal voltage
change has shown that SD-card does not always get recognized.

Linux driver use an enable_time of 400us for LDO regulators. Apply a
500us delay when a LDO regulator is enabled to fix possible issues.

Fixes: 94afc1cb ("power: regulator: rk8xx: update the driver for rk808 and rk818")
Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by:  <elaine.zhang&lt;elaine.zhang@rock-chips.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>