ARM: 7211/1: smp_twd: get the rate from a clock

This break-out from Colin Cross' cpufreq-aware TWD patch will
optionally retrieve the clock rate of the TWD from an external
clock. A variant of this patch has been proposed by Rob Herring
as well.

The basic idea is to avoid recalibrating the rate of the clock
at boot if the platform already know what rate the clock to the
TWD block has.

ChangeLog v1->v2: added clk_[prepare|unprepare] calls.

[Broke out of larger SMP TWD patch]

Signed-off-by: Colin Cross <ccross@android.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Rob Herring <rob.herring@calxeda.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>