MIPS: BCM63XX: add support for BCM3368 Cable Modem

MIPS: BCM63XX: add support for BCM3368 Cable Modem

The Broadcom BCM3368 Cable Modem SoC is extremely similar to the
existing BCM63xx DSL SoCs, in particular BCM6358, therefore little effort
in the existing code base is required to get it supported. This patch adds
support for the following on-chip peripherals:

- two UARTS
- GPIO
- Ethernet
- SPI
- PCI
- NOR Flash

The most noticeable difference with 3368 is that it has its peripheral
register at 0xfff8_0000 we check that separately in ioremap.h. Since
3368 is identical to 6358 for its clock and reset bits, we use them
verbatim.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Cc: linux-mips@linux-mips.org
Cc: cernekee@gmail.com
Cc: jogo@openwrt.org
Patchwork: https://patchwork.linux-mips.org/patch/5499/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>