ARM: mstar: Add binding details for mstar,l3bridge

This adds a YAML description of the l3bridge node needed by the
platform code for the MStar/SigmaStar Armv7 SoCs.

Signed-off-by: Daniel Palmer <daniel@0x0f.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>