ARM: BCM5301X: Add basic dts for BCM53573 based Tenda AC9

BCM53573 seems to be low priced alternative for Northstar chipsts. It
uses single core Cortex-A7 and doesn't have SDU or local (TWD) timer. It
was also stripped out of independent SPI controller and 2 GMACs.

DTS for Tenda AC9 isn't completed yet. It misses e.g. switch entry (we
still need some b53 fixes) and probably some clocks. It adds support for
basic features however and can be improved later.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>