ARM: 6277/1: mach-shmobile: Allow users to select HZ, default to 128

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: Magnus Damm <damm@opensource.se>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>