x86/resctrl: Promote resctrl_get_domain_from_cpu() to a header

resctrl_get_domain_from_cpu() is useful to MPAM before arm64 links
with resctrl. To keep the series simple, promote
resctrl_get_domain_from_cpu() to a header file.

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