x86/resctrl: Add weak resctrl_arch_find_domain()

resctrl_arch_find_domain() does nothing clever, and only has one user
in the fs code, (and none in the x86 arch code).

Provide a weak version that does the necessary. Future rework
should fold out resctrl_arch_find_domain() as an arch thing completely.

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

--
Future rework? mpam can't yet be bothered to sort the domain list by
id. Core code should do it. Once that is done, the x86-smarts of 'hey,
this is a sorted list', can move in here.