arm: mxs: fix comments in arch_cpu_init to match the code

The comment says to clear the bypass bit, but in fact it sets it, thus
selecting ref_xtal. And the next line of code does not set the divider
to 12, but to (the reset value of) 1.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>