drivers: ssd16xx: Implement basic read support

Read operations only work on SPI controllers that support half
duplex. To enable read operations, set the device mode to half duplex
by adding the following to the device's DTB node:

  duplex = <SPI_HALF_DUPLEX>;

If read requests will fail with -ENOTSUP if the device isn't in half
duplex mode.

Signed-off-by: Andreas Sandberg <andreas@sandberg.uk>