ARM: BCM5301X: Add DT entry for SPI controller and NOR flash

Controller is present on every BCM4708* board but only few devices have
serial flash attached so mark it as disabled by default.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>