OMAP2/3 clock: clean up omap*_clk_arch_init()

In the OMAP3xxx clock code, remove the #ifdef CONFIG_ARCH_OMAP3 in
clock34xx.c, since this file is only compiled for OMAP3xxx builds.  Also,
rename omap2_clk_arch_init in this file to omap3xxx_clk_arch_init() to
pave the way for multi-OMAP kernels.  Ensure that it is not executed
on non-OMAP3xxx systems.

In the OMAP2xxx clock code, rename omap2_clk_arch_init in this file to
omap2xxx_clk_arch_init() to pave the way for multi-OMAP kernels.
Ensure that it is not executed on non-OMAP2xxx systems.

Signed-off-by: Paul Walmsley <paul@pwsan.com>