Uwe reported a build failure when targetting a NOMMU platform with my
recent prefetch changes:

  arch/arm/lib/changebit.S: Assembler messages:
  arch/arm/lib/changebit.S:15: Error: architectural extension `mp' is
			not allowed for the current base architecture

This is due to use of the .arch_extension mp directive immediately prior
to an ALT_SMP(...) instruction. Whilst the ALT_SMP macro will expand to
nothing if !CONFIG_SMP, gas will still choke on the directive.

This patch fixes the issue by only emitting the sequence (including the
directive) if CONFIG_SMP=y.
Tested-by: NUwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: NWill Deacon <will.deacon@arm.com>
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>