cc2538: spi: Add format configuration options

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
cc2538: spi: Add enable and disable functions

This makes it possible to reduce the power consumption when the SPI is unused.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>