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>

arch/powerpc/lib/bootm.c

@@ -30,7 +30,6 @@
 
 DECLARE_GLOBAL_DATA_PTR;
 
-extern ulong get_effective_memsize(void);
 static ulong get_sp (void);
 extern void ft_fixup_num_cores(void *blob);
 static void set_clocks_in_mhz (bd_t *kbd);