spi: dw: Allow interface drivers to limit data I/O to word sizes

The commit dd114443 ("spi: dw-spi: Convert 16bit accesses to 32bit
accesses") changed all 16bit accesses in the DW_apb_ssi driver to 32bit.
This, unfortunately, breaks data register access on picoXcell, where the
DW IP needs data register accesses to be word accesses (all other
accesses appear to be OK).

This change introduces a new master variable to allow interface drivers
to specify that 16bit data transfer I/O is required.  This change also
introduces the ability to set this variable via device tree bindings in
the MMIO interface driver.  Both the core and the MMIO interface driver
default to the current 32bit behaviour.

Before this change, on a picoXcell pc3x3:
 spi_master spi32766: interrupt_transfer: fifo overrun/underrun
 m25p80 spi32766.0: error -5 reading 9f
 m25p80: probe of spi32766.0 failed with error -5

After this change:
 m25p80 spi32766.0: m25p40 (512 Kbytes)

Fixes: dd114443 ("spi: dw-spi: Convert 16bit accesses to 32bit accesses")
Signed-off-by: Michael van der Westhuizen <michael@smart-africa.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>