MIPS: BCM47XX: Clean up nvram header

1) Move private defines to the .c file
2) Move SPROM helper to the sprom.c
3) Drop unused code
4) Rename magic to the NVRAM_MAGIC
5) Add const to the char pointer we never modify

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Acked-by: Hauke Mehrtens <hauke@hauke-m.de>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/8289/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>