qemu: common: Add support for dynamic mtdparts for the platform

Add support for setting the default values for mtd partitions on the
platform. This would be used for updating the firmware image using
uefi capsule update with the dfu mtd backend driver.

Currently, values have been defined for the qemu arm64 platform, with
default values defined for the mtd partitions based on the NOR
flash. This can be subsequently extended for other qemu architectures
which need mtdparts set.

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