spi: pxa2xx: Introduce special type for Merrifield SPIs

Intel Merrifield SPI is actually more closer to PXA3xx. It has extended FIFO
(32 bytes) and additional registers to get or set FIFO thresholds.

Introduce new type for Intel Merrifield SPI host controllers and handle bigger
FIFO size.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20210510124134.24638-15-andriy.shevchenko@linux.intel.com


Signed-off-by: Mark Brown <broonie@kernel.org>