Si4703: Add powerUp() and powerDown()

[getSurreal]

Use the DISABLE register to shutdown or power up the radio for the Si4703.

[mathertel]

Good point for clarification:

With some chips that support multiple modes the power up should be handles when specifying the mode in setBand and power situation will be handled according the following schema:

• The init() function establishes communication with the radio chip. As a result the radio chip will not yet work and is in power down state.
• The setBand() function will configure the radio chip and start operation. The radio chip will be in power up state.
• The term() function will stop any operation. The radio chip will be in power down state. Communication with the chip will still work.

To implement a "standby" functionality setBand() for "on" and term() for "standby" should be used.

This is verified for SI4721 for now.