ARM: 8573/1: domain: move {set,get}_domain under config guard

Recursive undefined instrcution falut is seen with R-class taking an
exception. The reson for that is __show_regs() tries to get domain
information, but domains is not available on !MMU cores, like R/M
class.

Fix it by puting {set,get}_domain functions under CONFIG_CPU_CP15_MMU
guard and providing stubs for the case where domains is not supported.

Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>