arm: Add support for the Broadcom Northstar SoCs

arm: Add support for the Broadcom Northstar SoCs

The original Northstar is an ARM SoC series that comprise
BCM4709x and BCM5301x and uses a dual-core Cortex A9, the
global timer and a few other things.

This series should not be confused with North Star Plus
(NSP) which is partly supported by U-Boot already.

The SoC is well supported by the Linux kernel and OpenWrt
as it is used in many routers.

Since we currently don't need any chip-specific quirks
and can get the system up from just the device tree, a
mach-* directory doesn't even need to be added, just
some small Kconfig fragments.

Cc: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>