staging: vchiq_arm: Include linux/mm.h

vchiq_2835_arm.c uses functions and macros like put_page() or
PAGE_ALIGN(). So we better include the necessary header.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>