arm: zynq: Enable DM_ETH and DM_MMC only if subsystem is enabled

Do not enable DM_ETH/MMC if subsystems are not enabled.
This saves memory for memory constrained boards like cse.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>

arch/arm/Kconfig

@@ -779,10 +779,10 @@ config ARCH_ZYNQ
 	select SPL_BOARD_INIT if SPL
 	select SPL_OF_CONTROL if SPL
 	select DM
-	select DM_ETH
+	select DM_ETH if NET
 	select DM_GPIO
 	select SPL_DM if SPL
-	select DM_MMC
+	select DM_MMC if MMC
 	select DM_SPI
 	select DM_SERIAL
 	select DM_SPI_FLASH