spi: spi-geni-qcom: Add support to specify RT priority to framework

The SPI framework creates a worker thread for each controller(master)
which can be spawned with "real-time" priority. Allow specifying this as
an optional DT param for the master.
Also pass up any errors encountered during FIFO message prepare stage to
the framework.

Change-Id: I2bde04ba83ecf690802e8ebbd6e41c93b06b2cfd
Signed-off-by: Girish Mahadevan <girishm@codeaurora.org>