configs: Add rk3502.config

Build command: ./make.sh rk3502

Signed-off-by: Joseph Chen <chenjh@rock-chips.com>
Change-Id: Iff8a2c97d4456e755ca5f84790994711bc009f40

configs/rk3502.config

+CONFIG_BASE_DEFCONFIG="rk3506_defconfig"
+# CONFIG_CMD_MII is not set
+# CONFIG_CMD_NET is not set
+# CONFIG_CMD_PXE is not set
+# CONFIG_CMD_TFTP_BOOTM is not set
+# CONFIG_CMD_TFTP_FLASH is not set
+# CONFIG_DISPLAY is not set
+# CONFIG_DM_ETH is not set
+# CONFIG_DM_ETH_PHY is not set
+# CONFIG_DM_VIDEO is not set
+# CONFIG_I2C_EDID is not set
+# CONFIG_NET is not set
+# CONFIG_VIDEO_BRIDGE is not set
+# CONFIG_VIDEO is not set