MIPS: BCM47xx: Move NVRAM header to the include/linux/.

There are two reasons for having this header in the common place:
1) Simplifying drivers that read NVRAM entries. We will be able to
   safely call bcm47xx_nvram_* functions without #ifdef-s.
2) Getting NVRAM driver out of MIPS arch code. This is needed to support
   BCM5301X arch which also requires this NVRAM driver. Patch for that
   will follow once we get is reviewed.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Acked-by: Hauke Mehrtens <hauke@hauke-m.de>
Cc: linux-mips@linux-mips.org
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Paul Walmsley <paul@pwsan.com>
Cc: linux-soc@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/8619/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>