MAINTAINERS: Update resctrl entry

MAINTAINERS: Update resctrl entry

The x86 maintainers handle the resctrl filesystem and x86 architectural
resctrl code. Even so, the x86 maintainers are not part of the resctrl
section and not returned when scripts/get_maintainer.pl is run on resctrl
filesystem code. With patches flowing via x86 maintainers resctrl should
also ensure it follows the tip rules.

Add the x86 maintainer alias, x86@kernel.org, to the resctrl section to
ensure x86 maintainers are included in associated resctrl submissions.

Add a reference to the tip tree handbook to make it clear which rules
resctrl follows.

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://patch.msgid.link/4c14dd82e81737c6413e10fe097475b1cc0886fc.1775576382.git.reinette.chatre@intel.com