[ARM] 3675/2: Preparing for AT91SAM926 support

[ARM] 3675/2: Preparing for AT91SAM926 support

Patch from Andrew Victor

This prepares the way for adding support for the new Atmel AT91SAM926x
processors.

Major changes:
- Rename time.c to at91rm9200_time.c
- Rename common.c to at91rm9200.c
- Introduce ARCH_AT91, of which ARCH_AT91RM9200, ARCH_AT91SAM9260 and
ARCH_AT91SAM9261 are dependent.

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