spi: Broadcom BRCMSTB, NSP, NS2 SoC bindings

Added device tree bindings documentation for BRCMSTB, NSP, NS2 iProc
SoCs supported by spi-bcm-qspi, spi-brcmstb-qspi and spi-iproc-qspi driver.

Signed-off-by: Kamal Dasu <kdasu.kdev@gmail.com>
Signed-off-by: Yendapally Reddy Dhananjaya Reddy <yendapally.reddy@broadcom.com>
Signed-off-by: Mark Brown <broonie@kernel.org>