OMAP3: Correct get_sdr_cs_offset mask

The function get_sdr_cs_offset reads the CS_CFG register in the SDRC
to determine where CS1 is mapped to.  make_cs1_contiguous() will set
CS1 to follow after CS0.  The CS_CFG register has values in bits 9:8
and 3:0 but we had erroneously been testing 5:4 and 3:0 resulting in
incorrect offsets on platforms with less than 128MB as 3:0 describe
128MB hunks and 9:8 describe 32MB offsets after the 128MB hunk.

Tested-by: Grant Erickson <marathon96@gmail.com>
Signed-off-by: Tom Rini <trini@ti.com>