x86: make 'mem=' option work for efi platform

x86: make 'mem=' option work for efi platform

Current mem boot option only can work for non efi environments.  If the
user specifies add_efi_memmap, it cannot work for efi environment.

In the efi environment, we call e820_add_region() to add the memory map.
So we can modify __e820_add_region() and the mem boot option can work for
an efi environment.

Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Matt Fleming <matt.fleming@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>