u-boot: change SRAM settings

SDRAM Configuration Register 0x1400, set Refresh to 0x2000 was 0x3fff.
SDRAM Timin (High) Register 0x140C, set tRFC to 0x4F was 0x3A.

Signed-off-by: Mario Schuknecht <mario.schuknecht@dresearch-fe.de>