MIPS: BCM47xx: Fetch board info directly in callback function

MIPS: BCM47xx: Fetch board info directly in callback function

This drops another symbol dependency between setup.c and sprom.c which
will allow us to make SPROM code a separated module (and share it with
ARM).
Patch tested on Linksys WRT300N V1.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Cc: Hauke Mehrtens <hauke@hauke-m.de>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/11360/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>