Currently this looks up the frequency in the global singleton that must
be initialized by the per-chip RCC implementation. At present, this is
only done for the L0 family of chips.
Adds RccPeripheral trait for peripherals implementing clock enable and reset for a given peripheral.
Add macro table generting implementations of RccPeripheral for peripherals with clock set, currently restricted to SPI.
* SPI write v2 was hanging in write due to an infinite loop
* SPI word write was not followed by a read back
The u8 and u16 write/read logic have been refactored into write_word and
read_word.
