ARM: rpc: autoselect CPU_SA110

ARCH_RPC no longer supports other CPUs aside from StrongARM110,
so we can make the option implicitly selected by the platform
and no longer give the option of building a kernel without CPU
support.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Russell King <linux@arm.linux.org.uk>