cpu/gd32v: add periph_rtt support

[gschorcht]

Contribution description

This PR provides the periph_rtt support and is one of a bunch of PRs that complete the peripheral drivers for GD32VF103.

Testing procedure

tests/periph_rtt should work and give the following results:

Help: Press s to start test, r to print it is ready
START
main(): This is RIOT! (Version: 2023.04-devel-199-g2d429-cpu/gd32v/periph_rtt)

RIOT RTT low-level driver test
RTT configuration:
RTT_MAX_VALUE: 0xffffffff
RTT_FREQUENCY: 32768

Testing the tick conversion
Trying to convert 1 to seconds and back
Trying to convert 256 to seconds and back
Trying to convert 65536 to seconds and back
Trying to convert 16777216 to seconds and back
Trying to convert 2147483648 to seconds and back
All ok

Initializing the RTT driver
This test will now display 'Hello' every 5 seconds

RTT now: 148
Setting initial alarm to now + 5 s (163988)
rtt_get_alarm() PASSED
Done setting up the RTT, wait for many Hellos
{ "threads": [{ "name": "idle", "stack_size": 256, "stack_used": 216 }]}
{ "threads": [{ "name": "main", "stack_size": 1280, "stack_used": 480 }]}
Hello
Hello
Hello
Hello

Issues/PRs references

[riot-ci]

Murdock results

✔️ PASSED

4383e0a cpu/gd32v: fix periph_flashpage doc group

Success	Failures	Total	Runtime
6796	0	6796	09m:04s

Artifacts

• Documentation preview

[benpicco]

looks good to me!

[benpicco]

bors merge

[bors]

Build succeeded:

• Murdock
• static-tests
• tools-build-success