ARM: 9280/1: mm: fix warning on phys_addr_t to void pointer assignment

zero_page is a void* pointer but memblock_alloc() returns phys_addr_t type
so this generates a warning while using clang and with -Wint-error enabled
that becomes and error. So let's cast the return of memblock_alloc() to
(void *).

Cc: <stable@vger.kernel.org> # 4.14.x +
Fixes: 340a9828 ("ARM: 9266/1: mm: fix no-MMU ZERO_PAGE() implementation")
Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>