boards/arduino-due: allow changing frequency

[fjmolinas]

Contribution description

This PR allows overriding the default value for the rtt frequency, it's one I missed in #14303.

Testing procedure

Sadly I do not have a BOARD to test, but this should be the testing procedure:

• tests/periph_rtt should still pass on sam3 boards with different frequencies

• BOARD=arduino-due make -C tests/periph_rtt flash test --no-print-directory -j3

• CFLAGS+=-DRTT_FREQUENCY=1024 BOARD=arduino-due make -C tests/periph_rtt clean flash test --no-print-directory -j3

Although IMO a green Murdock should be enough.

Issues/PRs references

Realized I missed this sam* family while looking at #16254

[maribu]

The test fails (also in master) with Error: rtt_get_alarm() not working. Looks like the increase test coverage of that test found another bug I didn't notice before.

[maribu]

After rebasing on top of #16258 I get:

2021-03-30 14:58:17,181 # Help: Press s to start test, r to print it is ready
s
2021-03-30 14:58:18,502 # START
2021-03-30 14:58:18,511 # main(): This is RIOT! (Version: 2021.04-devel-1176-gc70d9e-fjmolinas)
2021-03-30 14:58:18,511 # 
2021-03-30 14:58:18,514 # RIOT RTT low-level driver test
2021-03-30 14:58:18,516 # RTT configuration:
2021-03-30 14:58:18,517 # RTT_MAX_VALUE: 0xffffffff
2021-03-30 14:58:18,519 # RTT_FREQUENCY: 1024
2021-03-30 14:58:18,520 # 
2021-03-30 14:58:18,521 # Testing the tick conversion
2021-03-30 14:58:18,524 # Trying to convert 1 to seconds and back
2021-03-30 14:58:18,528 # Trying to convert 256 to seconds and back
2021-03-30 14:58:18,532 # Trying to convert 65536 to seconds and back
2021-03-30 14:58:18,536 # Trying to convert 16777216 to seconds and back
2021-03-30 14:58:18,540 # Trying to convert 2147483648 to seconds and back
2021-03-30 14:58:18,541 # All ok
2021-03-30 14:58:18,541 # 
2021-03-30 14:58:18,544 # Initializing the RTT driver
2021-03-30 14:58:18,548 # This test will now display 'Hello' every 5 seconds
2021-03-30 14:58:18,549 # 
2021-03-30 14:58:18,550 # RTT now: 3
2021-03-30 14:58:18,552 # Setting initial alarm to now + 5 s (5123)
2021-03-30 14:58:18,555 # rtt_get_alarm() PASSED
2021-03-30 14:58:18,559 # Done setting up the RTT, wait for many Hellos
2021-03-30 14:58:24,372 # Hello
2021-03-30 14:58:30,197 # Hello
2021-03-30 14:58:36,024 # Hello
2021-03-30 14:58:41,847 # Hello
2021-03-30 14:58:47,671 # Hello

So timing is off quite a bit, but so it is on master. Hence, IMO the PR delivers what is promised. The bugs in the RTT are preexisting and unrelated to this PR.