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

arch/arm/cpu/armv7/mx6/ddr.c

@@ -357,8 +357,8 @@ void mx6_dram_cfg(const struct mx6_ddr_sysinfo *sysinfo,
 		txs = DIV_ROUND_UP(170000, clkper) - 1;
 		break;
 	case 4: /* 4Gb per chip */
-		trfc = DIV_ROUND_UP(260000, clkper) - 1;
-		txs = DIV_ROUND_UP(270000, clkper) - 1;
+		trfc = DIV_ROUND_UP(300000, clkper) - 1;
+		txs = DIV_ROUND_UP(310000, clkper) - 1;
 		break;
 	case 8: /* 8Gb per chip */
 		trfc = DIV_ROUND_UP(350000, clkper) - 1;