sunxi: fix DRAM gate/reset sequence of H6

Currently the DRAM bus gate and reset is changed at the same time in
H6 DRAM initialization code, which disobeys the user manual's
programming guide.

Fix the sequence by follow the sequence suggested by the user manual
(ungate the bus clock after release the reset signal).

By some experiments it seems to fix the DRAM size detection failure that
rarely happens.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>