MIPS: BCM47xx: Move filling most of SPROM to the generic function

This simplifies code a lot by dropping many per-revision-group
functions. There are still some paths left that use uncommon NVRAM read
helpers or fill arrays. They will need to be handled in separated patch.

I've tested this (by printing SPROM content) for regressions on:
1) BCM4704 (SPROM revision 2)
2) BCM4706 (SPROM revision 8 plus 11 & 9 on extra WiFi cards)
The only difference is not reading board_type from SPROM rev 11 which is
unsupported and treated as rev 1. This change for rev 1 is expected.
Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: Hauke Mehrtens <hauke@hauke-m.de>
Cc: Jonas Gorski <jonas.gorski@gmail.com>
Patchwork: https://patchwork.linux-mips.org/patch/9660/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>