imx: serial: use rational library function

for calculation of numerator and denominator
used in baud rate setting, use generic library function
for optimum settings.

Signed-off-by: Oskar Schirmer <os@emlix.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>