Remove the arch/arm/plat-samsung/include/plat/samsung-time.h header and
move the contents to common.h headers in mach-s3c24xx and mach-s3c64xx.
The definition of declared functions is already in common.c in mach
directories, so it is logically to put declaration next to them.

This is also one step further towards removal of plat-samsung directory
and it fixes W=1 build warnings:

    arch/arm/mach-s3c64xx/common.c:174:13: warning: no previous prototype for 'samsung_set_timer_source' [-Wmissing-prototypes]
    arch/arm/mach-s3c64xx/common.c:180:13: warning: no previous prototype for 'samsung_timer_init' [-Wmissing-prototypes]
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Tomasz Figa <tomasz.figa@gmail.com>