i.MX28: Make the stabilization delays shorter

Cut down the VDDIO/VDDA regulator stabilization delays to 500 uS. That should be
enough according to the datasheet and bootlets.

Signed-off-by: Marek Vasut <marek.vasut@gmail.com>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Detlev Zundel <dzu@denx.de>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Robert Deliën <robert@delien.nl>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: Matthias Fuchs <matthias.fuchs@esd.eu>