This patch implements the basic support the last of the FLEXCOMM modes,
Serial Peripheral Interface, in a simple blocking mode with busy wait,
which is enough to test all the SPI functionality end-to-end.
Tested reading and writing registers on a SPI peripheral, and checked
with the oscilloscope that the frequencies were as expected.
Results from `tests/periph_spi`:
```
> init 0 0 2 -1 0
SPI_DEV(0) initialized: mode: 0, clk: 2, cs_port: -1, cs_pin: 0
> bench
1 - write 1000 times 1 byte: 16002 16009
2 - write 1000 times 2 byte: 18001 18008
3 - write 1000 times 100 byte: 802000 802007
4 - write 1000 times 1 byte to register: 24003 24010
5 - write 1000 times 2 byte to register: 26001 26008
6 - write 1000 times 100 byte to register: 810001 810008
7 - read 1000 times 2 byte: 23003 23009
8 - read 1000 times 100 byte: 807002 807009
9 - read 1000 times 2 byte from register: 32002 32009
10 - read 1000 times 100 byte from register: 816002 816009
11 - transfer 1000 times 2 byte: 23003 23009
12 - transfer 1000 times 100 byte: 807003 807010
13 - transfer 1000 times 2 byte to register: 32003 32009
14 - transfer 1000 times 100 byte to register:816002 816009
15 - acquire/release 1000 times: 7222 7228
-- - SUM: 5059250 5059351
```
