MIPS: Prune some target specific code out of prom.c

MIPS: Prune some target specific code out of prom.c

This code is not common enough to be in a shared file.  It is also not
used by any existing boards, so just remove it.

[ralf@linux-mips.org: Dropped removal of irq_create_of_mapping which was
already removed by abd2363f6a5f1030b935e0bdc15cf917313b3b10
[irq_domain/mips: Allow irq_domain on MIPS].  Moved device_tree_init() and
dependencies to its sole user, the XLP code.]

Signed-off-by: David Daney <david.daney@cavium.com>
Cc: linux-mips@linux-mips.org
Cc: devicetree-discuss@lists.ozlabs.org
Cc: grant.likely@secretlab.ca
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/2946/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>