ARM: dts: BCM5301X: convert to iProc QSPI

The iproc-qspi driver is the SPI driver that should be used going
forward.  Modify the SPI DT entry to use this driver, and add an entry
in the bcm953012k DTS file to enable the SPI.

Tested on the bcm953012k board.

Signed-off-by: Jon Mason <jon.mason@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>