ARM: mm: fix compilation warning

This fixes the following compilation warning:

  arch/arm/mm/proc-v7-2level.S: Assembler messages:
  arch/arm/mm/proc-v7-2level.S:176: Warning: .previous without corresponding .section; ignored

Signed-off-by: Vincent Stehlé <vincent.stehle@freescale.com>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: trivial@kernel.org