leds: spi-byte: add single byte SPI LED driver

This driver adds support for simple SPI based LED controller which use
only one byte for setting the brightness.

Signed-off-by: Christian Mauderer <oss@c-mauderer.de>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>