spi: dw: don't use mrst prefix anymore

Since driver is used on other platforms and debugfs stuff would be useful there
as well let's substitute mrst_ by dw_ where it suits. Additionally let's use
SPI master device name when print registers dump.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>