ARM: 8407/1: switch_to: Remove finish_arch_switch

Fold finish_arch_switch() into switch_to(), in preparation for the
removal of the finish_arch_switch call from core sched code.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>