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>
diff --git a/common/cmd_log.c b/common/cmd_log.c
index 8164bdf..38d0f5e 100644
--- a/common/cmd_log.c
+++ b/common/cmd_log.c
@@ -52,7 +52,7 @@
 
 unsigned long __logbuffer_base(void)
 {
-	return CONFIG_SYS_SDRAM_BASE + gd->ram_size - LOGBUFF_LEN;
+	return CONFIG_SYS_SDRAM_BASE + get_effective_memsize() - LOGBUFF_LEN;
 }
 unsigned long logbuffer_base(void)
 __attribute__((weak, alias("__logbuffer_base")));