ARM: S5PC1XX: Remove definitions deleted by previous clksrc changes

Remove the definitions we've deleted in the previous updates to the
clksrc_clk for arch/arm/plat-s5pc1xx/include/plat/regs-clock.h.

Added comments about the removal to the clock header since we only need
these defines in one place (and they've now been removed there) we get
rid of them from the header.

Signed-off-by: Ben Dooks <ben-linux@fluff.org>