spi: Convert DW SPI binding to DT schema

Modern device tree bindings are supposed to be created as YAML-files
in accordance with dt-schema. This commit replaces two DW SPI legacy
bare text bindings with YAML file. As before the bindings file states
that the corresponding dts node is supposed to be compatible either
with generic DW APB SSI controller or with Microsemi/Amazon/Renesas/Intel
vendors-specific controllers, to have registers, interrupts and clocks
properties. Though in case of Microsemi version of the controller
there must be two registers resources specified. Properties like
clock-names, reg-io-width, cs-gpio, num-cs, DMA and slave device
sub-nodes are optional.

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Reviewed-by: Rob Herring <robh@kernel.org>
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: Feng Tang <feng.tang@intel.com>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: linux-mips@vger.kernel.org
Link: https://lore.kernel.org/r/20200529182544.9807-1-Sergey.Semin@baikalelectronics.ru


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