imx: mx6: ddr correct tRFC and tXS

To Chip density 4Gb, tRFC should be 300ns, see
"Table 61 — Refresh parameters by device density" of JESD79-3E.
tXS(min) is max(5nCK, tRFC(min) + 10ns).

Signed-off-by: Peng Fan <Peng.Fan@freescale.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Tim Harvey <tharvey@gateworks.com>
Reviewed-by: Stefano Babic <sbabic@denx.de>