mips: fix HIGHMEM initialization

mips: fix HIGHMEM initialization

Commit 6faea3422e3b ("arch, mm: streamline HIGHMEM freeing") overzealously
removed mem_init_free_highmem() function that beside freeing high memory
pages checked for CPU support for high memory as a prerequisite.

Partially restore mem_init_free_highmem() with a new highmem_init() name
and make it discard high memory in case there is no CPU support for it.

Link: https://lkml.kernel.org/r/20251231105701.519711-1-rppt@kernel.org
Fixes: 6faea3422e3b ("arch, mm: streamline HIGHMEM freeing")
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Reported-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Cc: Chris Packham <chris.packham@alliedtelesis.co.nz>
Cc: Hauke Mehrtens <hauke@hauke-m.de>
Cc: Jonas Jelonek <jelonek.jonas@gmail.com>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: Thomas Gleinxer <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>