spi: omap2-mcspi: Support divide-by-n clock dividers

Currently a divide-by-2^n clock is used, causing a very coarse
clock selection, i.e. a 10MHz device will need to use a 6MHz
clock. The McSPI can also use a divide-by-n clock, this patch
adds support for selecting that when possible.

Signed-off-by: Stefan Sørensen <stefan.sorensen@spectralink.com>
Signed-off-by: Mark Brown <broonie@linaro.org>