common: Add get_effective_memsize() to memsize.c

This function has been around for powerpc. It is used for systems with
memory more than CONFIG_MAX_MEM_MAPPED. In case of non-contiguous memory,
this feature can limit U-boot to one block without going over the limit.

Signed-off-by: York Sun <yorksun@freescale.com>
Acked-by: Albert ARIBAUD <albert.u.boot@aribaud.net>

include/common.h

@@ -461,6 +461,7 @@ void	api_init (void);
 
 /* common/memsize.c */
 long	get_ram_size  (long *, long);
+phys_size_t get_effective_memsize(void);
 
 /* $(BOARD)/$(BOARD).c */
 void	reset_phy     (void);