ARM: 6053/1: AT91: Save power by disabling the processor clock when CPU is idle

Disable the processor clock when the CPU is idle.

This saves much more power than merely entering 'Wait for Interrupt' mode.
Since JTAG-debugging doesn't work when the processor clock is switched off,
make it conditional on CONFIG_DEBUG_KERNEL.

Signed-off-by: Anders Larsen <al@alarsen.net>
Acked-by: Andrew Victor <avictor.za@gmail.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>