arm: multi_v7_defconfig: Enable initrd/initramfs support

Add CONFIG_BLK_DEV_INITRD to the defconfig to support
initramfs and initrd.

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>