powerpc: respect mem= setting for early memory limit setup

For those MMUs that have some form of bolt'd linear mapping (TLB)
required its rare that one ever sets mem= smaller than the size of that
mapping.

However, on Book-E 64 parts the initial linear mapping is quite large
(1G) so its quite reasonable that mem= is set smaller than that.

We need to parse the command line for mem= limit and constrain the
amount of memory we map initially by it if need be.
Signed-off-by: NKumar Gala <galak@kernel.crashing.org>

arch/powerpc/kernel/prom.c

@@ -737,12 +737,15 @@ void __init early_init_devtree(void *params)
 
 	of_scan_flat_dt(early_init_dt_scan_root, NULL);
 	of_scan_flat_dt(early_init_dt_scan_memory_ppc, NULL);
-	setup_initial_memory_limit(memstart_addr, first_memblock_size);
 
 	/* Save command line for /proc/cmdline and then parse parameters */
 	strlcpy(boot_command_line, cmd_line, COMMAND_LINE_SIZE);
 	parse_early_param();
 
+	/* make sure we've parsed cmdline for mem= before this */
+	if (memory_limit)
+		first_memblock_size = min(first_memblock_size, memory_limit);
+	setup_initial_memory_limit(memstart_addr, first_memblock_size);
 	/* Reserve MEMBLOCK regions used by kernel, initrd, dt, etc... */
 	memblock_reserve(PHYSICAL_START, __pa(klimit) - PHYSICAL_START);
 	/* If relocatable, reserve first 32k for interrupt vectors etc. */