drivers/ieee802154: Switch MCR20A driver to new SPI API

And adding support for GPIO CS as well.
It looks like the driver could benefit from centralizing all SPI access
into a unique function, the protocol does not seem too convoluted to do
so, like CC2520 or CC1200.

Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>