qemu: common: Set dfu_alt_info variable for the platform

The dfu framework uses the dfu_alt_info environment variable to get
information that is needed for performing the firmware update. Add
logic to set the dfu_alt_info for the qemu arm64 platform to reflect
the two mtd partitions created for the u-boot env and the firmware
image. This can be subsequently extended for other qemu architectures
which need this variable set.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>