omap2 clock: Cleanup in clksel-related code; add sys_clkout2 divisor handling

Convert omap2_get_clksel to use void __iomem *.  Add
omap2_divisor_to_clksel(), to convert clock divisors into appropriate
register bit field values.  Use non-shifted register bit field masks
in clksel code, rather than masks that have been preshifted down to
bit 0 -- this simplifies existing code and facilitates the use of
symbolic constants from the recent PRCM cleanup.  Also add code to
properly handle divisors for sys_clkout2 on OMAP2420.  Previously
this clock's divisor settings were ignored.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>