OMAP2/3/4 clock: move clksel clock functions into mach-omap2/clkt_clksel.c

Move all clksel-related clock functions from mach-omap2/clock.c to
mach-omap2/clkt_clksel.c.  This is intended to make the clock code
easier to understand, since all of the functions needed to manage
clksel clocks are now located in their own file, rather than being
mixed with other, unrelated functions.

Clock debugging is also now more finely-grained, since the DEBUG macro
can now be defined for clksel clocks alon.  This should reduce
unnecessary console noise when debugging.

Also, if at some future point the mach-omap2/ directory is split
into OMAP2/3/4 variants, this clkt file can be moved to the plat-omap/
directory to be shared.

Thanks to Alexander Shishkin <virtuoso@slind.org> for his comments to
improve the patch description.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Alexander Shishkin <virtuoso@slind.org>