[ARM] 4392/2: Do not corrupt the SP register in compressed/head.S

ARMv7 support code requires a valid stack for saving/restoring
registers as the whole D-cache flushing function is more complex. This
patch ensures that the SP register is not corrupted.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>