x86/resctrl: Merge arch specific headers

Having a separate resctrl_sched.h for one function starts to get tricky
onve we move resctrl out to /fs/. This forces other architecture's to have
the same header file layout, or face the include-order nightmare.

Move resctrl_sched_in() into asm/resctrl.h.

While we're at it, use IS_ENABLED() instead of #ifdeffery, and give
__resctrl_sched_in() the inline it richly deserves.

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