MIPS: fix memory setup for platforms with PHYS_OFFSET != 0

MIPS: fix memory setup for platforms with PHYS_OFFSET != 0

For platforms, which use a PHYS_OFFSET != 0, symbol _end also
contains that offset. So when calling memblock_reserve() for
reserving kernel the size argument needs to be adjusted.

Fixes: bcec54bf3118 ("mips: switch to NO_BOOTMEM")
Acked-by: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: Thomas Bogendoerfer <tbogendoerfer@suse.de>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: James Hogan <jhogan@kernel.org>
Cc: linux-mips@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: stable@vger.kernel.org # v4.20+