spi: mxc_spi: Update pre and post divider algorithm

spi: mxc_spi: Update pre and post divider algorithm

The spi clock divisor is of the form x * (2**y),  or  x  << y, where x is
1 to 16, and y is 0 to 15. Note the similarity with floating point numbers.
Convert the desired divisor to the smallest number which is >= desired divisor,
and can be represented in this form. The previous algorithm chose a divisor
which could be almost twice as large as needed.

Signed-off-by: Troy Kisky <troy.kisky@boundarydevices.com>
Signed-off-by: Dirk Behme <dirk.behme@gmail.com>