fs/resctrl: Add arch hook to set up the default CLOSID

Currently, the resctrl core code assumes that CLOSID 0 requires no
special initialisation before use.

This is true on x86, but may not be true on other architectures
(including Arm MPAM, in certain configurations).

Since the allocation of CLOSIDs can depend on parameters such as
resctrl filesystem mount options that are not available at arch driver
init time, an arch hook is needed to set up the default CLOSID when
the resctrl filesystem is mounted.

Add a suitable arch hook resctrl_arch_init_domains().

Wire it up to arch stubs that do nothing, for now.
(This is also the right thing for x86.)

No functional change.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>