mx5 clocks: Cleanup

Clean up the i.MX5 clock driver:
 - Use readl() and writel() instead of their __raw_ counterparts.
 - Use the clr/setbits_le32() family of macros rather than expanding code.
 - Use accessor macros for bit-fields instead of _MASK and _OFFSET.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Cc: Stefano Babic <sbabic@denx.de>