[ARM] 4646/1: AT91: configurable HZ, default to 128

This makes HZ configurable on AT91, following the model used on OMAP.

It defaults to a power of two on AT91rm9200 chips, avoiding rounding
errors which come from dividing a 32 KiHz clock to generate scheduler
irqs; and uses 100 on AT91sam926x chips, using MCK/16 (multi-MHZ).

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Acked-by: Remy Bhmer <linux@bohmer.net>
Acked-by: Andrew Victor <andrew@sanpeople.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>