xilinx: Enable SPI_FLASH_MTD by default

Provide access to qspi flash layout via mtd command.

Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/b3c4a3eddb71aab9535b034380d0dbae770828d4.1698837725.git.michal.simek@amd.com
diff --git a/configs/xilinx_zynq_virt_defconfig b/configs/xilinx_zynq_virt_defconfig
index 6145af6..c3ee9be 100644
--- a/configs/xilinx_zynq_virt_defconfig
+++ b/configs/xilinx_zynq_virt_defconfig
@@ -127,6 +127,7 @@
 CONFIG_SPI_FLASH_SST=y
 CONFIG_SPI_FLASH_WINBOND=y
 # CONFIG_SPI_FLASH_USE_4K_SECTORS is not set
+CONFIG_SPI_FLASH_MTD=y
 CONFIG_PHY_MARVELL=y
 CONFIG_PHY_MICREL=y
 CONFIG_PHY_MICREL_KSZ90X1=y