x86/platform/uv: Update ABI documentation of /sys/firmware/sgi_uv/

Update the ABI documentation to describe the sysfs interface provided by
the new uv_sysfs platform driver.

 [ bp: Merge in kernel-doc warning fixes, see second Link: below. ]

Signed-off-by: Justin Ernst <justin.ernst@hpe.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Steve Wahl <steve.wahl@hpe.com>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lkml.kernel.org/r/20201125175444.279074-5-justin.ernst@hpe.com
Link: https://lkml.kernel.org/r/20201130214304.369348-1-justin.ernst@hpe.com