[ARM] 3946/1: AT91: at91_arch_reset and at91_extern_irq

The external interrupt sources are different on the various AT91
processors.  This patch introduces the global 'at91_extern_irq' variable
that contains a bitset of the available external interrupt sources.

The processor reset mechanism also differs on the various AT91
processors.  This patch also adds a global 'at91_arch_reset' callback
(from system.h) into the processor-specific code to perform the reset.

Signed-off-by: Andrew Victor <andrew@sanpeople.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>