drivers: spi: introduce TI omap_mcspi

This patch adds the initial support for the OMAP Multi-Channel SPI. Some
things should be noted however:

- DMA xfers are not supported yet. Only PIO is supported as of now.
- Multi-Channel controller is not supported yet. Only single-channel
  controller mode is supported, this means that the controller can xfer
  messages with one slave at a time.

Signed-off-by: Amneesh Singh <a-singh7@ti.com>