clk: mxs: Fix invalid 32-bit access to frac registers
According to i.MX23 and i.MX28 reference manual [1],[2] the fractional
clock control register is 32-bit wide, but is separated in 4 parts.
So write instructions must not apply to more than 1 part at once.
The clk init for the i.MX28 violates this restriction and all the other
accesses on that register suggest that there isn't such a restriction.
This patch restricts the access to this register to byte instructions and
extends the comment in the init functions.
Btw the imx23 init now uses a R-M-W sequence just like imx28 init
to avoid any clock glitches.
The changes has been tested with a i.MX23 and a i.MX28 board.