MIPS: Fix build with CONFIG_PROC_VMCORE=y

Commit a94e4f24ec ("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>

arch/mips/kernel/setup.c

@@ -458,7 +458,7 @@ static int __init early_parse_elfcorehdr(char *p)
 
 	 for_each_memblock(memory, mem) {
 		unsigned long start = mem->base;
-		unsigned long end = mem->end;
+		unsigned long end = start + mem->size;
 		if (setup_elfcorehdr >= start && setup_elfcorehdr < end) {
 			/*
 			 * Reserve from the elf core header to the end of