spi/qspi: Add qspi flash controller

spi/qspi: Add qspi flash controller

The patch add basic support for the quad spi controller.

QSPI is a kind of spi module that allows single,
dual and quad read access to external spi devices. The module
has a memory mapped interface which provide direct interface
for accessing data form external spi devices.

The patch will configure controller clocks, device control
register and for defining low level transfer apis which
will be used by the spi framework to transfer data to
the slave spi device(flash in this case).

Test details:
-------------
Tested this on dra7 board.
Test1: Ran mtd_stesstest for 40000 iterations.
   - All iterations went through without failure.
Test2: Use mtd utilities:
  - flash_erase to erase the flash device
  - mtd_debug read to read data back.
  - mtd_debug write to write to the data flash.
 diff between the write and read data shows zero.

Acked-by: Felipe Balbi<balbi@ti.com>
Reviewed-by: Felipe Balbi<balbi@ti.com>
Signed-off-by: Sourav Poddar <sourav.poddar@ti.com>
Signed-off-by: Mark Brown <broonie@linaro.org>