spi: dw: Add Tx/Rx DMA properties

Since commit 22d48ad7 ("spi: dw: Add Elkhart Lake PSE DMA support")
the spi-dw-mid.c module supports a platform DMA engine handling the DW APB
SSI controller requests. Lets alter the DW SPI bindings file to accept the
Rx and Tx DMA line specifiers.

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Cc: Georgy Vlasov <Georgy.Vlasov@baikalelectronics.ru>
Cc: Ramil Zaripov <Ramil.Zaripov@baikalelectronics.ru>
Cc: Alexey Malahov <Alexey.Malahov@baikalelectronics.ru>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: Paul Burton <paulburton@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Allison Randal <allison@lohutok.net>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Gareth Williams <gareth.williams.jx@renesas.com>
Cc: linux-mips@vger.kernel.org
Link: https://lore.kernel.org/r/20200515104758.6934-2-Sergey.Semin@baikalelectronics.ru


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