add subticks to RP2040 port_get_raw_ticks

[timchinowsky]

The RP2040 port of monotonic_ns() only has millisecond precision - the subtick value is not used:

circuitpython/ports/raspberrypi/supervisor/port.c

Lines 268 to 271 in 781c577

	uint64_t port_get_raw_ticks(uint8_t *subticks) {
	uint64_t microseconds = time_us_64();
	return 1024 * (microseconds / 1000000) + (microseconds % 1000000) / 977;
	}

Added 7234375 to fill in the subtick field. Testing with this code to look at deltas between successive calls to monotonic_ns():

import time
def test_ns(n=200):
    t0 = time.monotonic_ns()
    for i in range(n):
        t1 = time.monotonic_ns()
        print(t1-t0)
        t0 = t1
test_ns()

Plotting the returned data from before and after the change:

Before the change, the data is granular to 1 ms, e.g.

976563
0
0
0
0
976562
0
0
0
0
976563

After, it does better:

244144
213613
183108
183108
213626
213626
213612
244144
152590
213626
244131

[tannewt]

Looks good! Thank you!