ARM: s5p: consolidate selection of timer register

s5p duplicates the runtime selection of the timer register three times.
Move this out into a separate function.

FIXME: It is unclear whether this code needs to support true runtime
selection of the timer register, or whether it can be selected once at
init time.

Acked-by: Kukjin Kim <kgene.kim@samsung.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>