omap3: Fix SPI registers on am33xx and am43xx

When the base registers are read from device tree the base is not
0x48030100 as the driver expects, but 0x48030000, resulting in
non functioning SPI. To deal with this, use same idea as how this
is done in the linux kernel (drivers/spi/spi-omap2-mcspi.c) and
add a structure with a field that is used to shift the registers
on these systems.

v2: Fixed commit subject line to correct cpu

Signed-off-by: Martin Hejnfelt <mh@newtec.dk>