Introduce SHMOBILE_TIMER_HZ for SH-Mobile.

Allow users to select HZ on their system to
minimize potential timer drift. Use 128 Hz as
default to work well with the 32768 Hz RCLK.
Signed-off-by: NMagnus Damm <damm@opensource.se>
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>