MIPS: Fix build with CONFIG_PROC_VMCORE=y

Commit a94e4f24 ("MIPS: init: Drop boot_mem_map") introduced a
reference to a non-existant "end" field in struct memblock_region.
Replace it with a sum of the base & size fields to fix builds with
CONFIG_PROC_VMCORE=y.

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Paul Burton <paul.burton@mips.com>