Documentation: filesystems: Add ARM specifics to resctrl_ui.txt

Now that resctrl can be supported on arm64 systems, add it to the
document.

Some features aren't supported on arm64 at all, but they also
aren't supported on all x86 systems, so this isn't called out.

There are no flags in arm64's /proc/cpuinfo, and no cmdline arguments
that affecting MPAM.

Signed-off-by: James Morse <james.morse@arm.com>