runners: xsdb: Add support for optional bl31, pdi, system.dtb arguments

This update enhances the runner script to optionally accept bl31, pdi,
and system.dtb as arguments. These inputs are not mandatory and will
only be used if explicitly provided. This allows for more flexible boot
configurations, especially useful when testing different firmware and
system setups without modifying the script.

Signed-off-by: Appana Durga Kedareswara rao <appana.durga.kedareswara.rao@amd.com>