[ARM] 3973/1: AT91: Serial driver compile fix

1. The CPU identification has moved from <system.h> to <cpu.h>.

2. AT91RM9200_BASE_US0 is only defined if we are compiling in support
for the AT91RM9200.  Therefore we need to replace the CONFIG_ARM ifdef
with CONFIG_ARCH_AT91RM9200

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