OMAP2xxx clock: move sys_clk code into mach-omap2/clkt2xxx_sys.c

Move the sys_clk clock functions from clock2xxx.c to
mach-omap2/clkt2xxx_sys.c.  This is intended to make the clock code
easier to understand, since all of the functions needed to manage the
sys_clk 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 the sys_clk clock alone.  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 placed in the mach-omap2xxx/
directory, rather than shared with other chip types that don't use
this clock type.

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>