ARM: mm: fix boot on SA1110 Assabet

Commit 83db0384

 (mm/ARM: use common help functions to free reserved
pages) broke booting on the Assabet by trying to convert a PFN to
a virtual address using the __va() macro.  This macro takes the
physical address, not a PFN.  Fix this.

Cc: <stable@vger.kernel.org> # 3.10
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>