spi: spi_qsd: Add slave support for QUP core

The QUP core can be programmed to operate as an SPI slave. This
change modifies the existing SPI master driver and adds the slave
functionality.

Change-Id: I73189339956e0fd52449391737fb1e616a6e0bb2
Signed-off-by: Dilip Kota <dkota@codeaurora.org>