powerpc: Move CONFIG_SYS_DDR_SIZE to CONFIG_SYS_SDRAM_SIZE

We have a number of CONFIG_SYS_xxx_SIZE options to describe the amount
main memory available.  Rework CONFIG_SYS_DDR_SIZE, which described a
size in number of MiB to use CONFIG_SYS_SDRAM_SIZE which is most often
used as a number of bytes.  Use shifts of this option when required.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/board/gdsys/mpc8308/sdram.c b/board/gdsys/mpc8308/sdram.c
index bfd55f5..47b8804 100644
--- a/board/gdsys/mpc8308/sdram.c
+++ b/board/gdsys/mpc8308/sdram.c
@@ -34,7 +34,7 @@
 static long fixed_sdram(void)
 {
 	immap_t *im = (immap_t *)CONFIG_SYS_IMMR;
-	u32 msize = CONFIG_SYS_DDR_SIZE * 1024 * 1024;
+	u32 msize = CONFIG_SYS_SDRAM_SIZE;
 	u32 msize_log2 = __ilog2(msize);
 
 	out_be32(&im->sysconf.ddrlaw[0].bar,