configs: Update starter_fs.py for latest Arm FS binaries.

configs: Update starter_fs.py for latest Arm FS binaries.

Updated the default kernel and root device names to match the latest
Arm full-system binaries available for download on the gem5 website.

Also added a command line option to allow the root device to be
specified as an optional command line argument.

Change-Id: I27f90ffaf0f4b35c5dcc4c22ac2fbd34f8a040a4
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/30814
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>