spi: add support for ARM PL022 SPI controller

This adds support for the ARM PL022 SPI controller for the standard
variant (0x00041022) which has a 16bit wide and 8 locations deep TX/RX
FIFO.

A few parts were borrowed from the Linux kernel driver.

Cc: Armando Visconti <armando.visconti@st.com>
Cc: Vipin Kumar <vipin.kumar@st.com>
Signed-off-by: Quentin Schulz <quentin.schulz@bootlin.com>