Handling of region-specific power limits

[plaes]

Now that support for PhyRxTx has gained support for limiting maximum power and dealing with board-specific antenna gain via #159, region-specific transmission power limits should be revisited (see also #150).
Also, all drivers in lora-phy have been fixed to clamp output power between supported values instead of raising Err(InvalidOutputPower).

Values for maximum transmission power for each region have been pulled from RP002-1.0.3 Regional Parameters.

Dynamic Channel plans (currently, the maximum EIRP value ( Region::get_dbm()) is 14dBm):

1. AS923: +16dBm
2. EU433: +12dBm
3. EU863-870: +16dBm
4. IN865-867: +30dBm

Fixed channel plans:

1. AU915-928: +30dBm - currently 21dBm
2. US902-928: +30dBm (?) - currently 21dBm

Unimplemented regions:

1. CN779-787: +12dBm
2. CN470-510: +19dBm
3. KR920-923: (depends on frequency: +12dBm if f < 922MHz), otherwise +14dBm
4. RU864-870: +16dBm