MIPS: BCM63XX: Add flash type detection

On BCM6358 and BCM6368 the attached flash type is exposed through a
bootstrapping register. Use it for auto detecting the flash type on
those and default to parallel flash for earlier SoCs.

Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: Maxime Bizon <mbizon@freebox.fr>
Cc: Florian Fainelli <florian@openwrt.org>
Cc: Kevin Cernekee <cernekee@gmail.com>
Patchwork: https://patchwork.linux-mips.org/patch/3954/


Reviewed-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>