rk3288: config change for enable dram capacity auto-detect.

Enable ROCKCHIP_SPL_BACK_TO_BROM and disable CONFIG_SPL_MMC_SUPPORT
to save memory in order to enable add source code for dram capacity
auto-detect.
Signed-off-by: NKever Yang <kever.yang@rock-chips.com>

configs/evb-rk3288_defconfig

@@ -64,3 +64,4 @@ CONFIG_SYSRESET=y
 CONFIG_USE_TINY_PRINTF=y
 CONFIG_CMD_DHRYSTONE=y
 CONFIG_ERRNO_STR=y
+CONFIG_ROCKCHIP_SPL_BACK_TO_BROM=y

configs/fennec-rk3288_defconfig

@@ -63,3 +63,4 @@ CONFIG_SYSRESET=y
 CONFIG_USE_TINY_PRINTF=y
 CONFIG_CMD_DHRYSTONE=y
 CONFIG_ERRNO_STR=y
+CONFIG_ROCKCHIP_SPL_BACK_TO_BROM=y

configs/miniarm-rk3288_defconfig

@@ -62,3 +62,4 @@ CONFIG_SYSRESET=y
 CONFIG_USE_TINY_PRINTF=y
 CONFIG_CMD_DHRYSTONE=y
 CONFIG_ERRNO_STR=y
+CONFIG_ROCKCHIP_SPL_BACK_TO_BROM=y

configs/popmetal-rk3288_defconfig

@@ -62,3 +62,4 @@ CONFIG_SYSRESET=y
 CONFIG_USE_TINY_PRINTF=y
 CONFIG_CMD_DHRYSTONE=y
 CONFIG_ERRNO_STR=y
+CONFIG_ROCKCHIP_SPL_BACK_TO_BROM=y