rp2/machine_rtc: Add initial support for RTC.

Initial support for machine.RTC on rp2 port. It only supports datetime()
method and nothing else. The method gets/returns a tuple of 8 items, just
like esp32 port, for example, but the usec parameter is ignored as the RP2
RTC only works up to seconds precision.

The Pico RTC isn't very useful as the time is lost during reset and there
seems to be no way to easily power up just the RTC clock with a low current
voltage, but still there seems to be use-cases for that, see issues #6831,
and a Thonny issue #1592. It was also requested for inclusion on v1.15
roadmap on #6832.

Signed-off-by: Krzysztof Adamski <k@japko.eu>

ports/rp2/CMakeLists.txt

@@ -86,6 +86,7 @@ set(MICROPY_SOURCE_PORT
     machine_i2c.c
     machine_pin.c
     machine_pwm.c
+    machine_rtc.c
     machine_spi.c
     machine_timer.c
     machine_uart.c
@@ -113,6 +114,7 @@ set(MICROPY_SOURCE_QSTR
     ${PROJECT_SOURCE_DIR}/machine_i2c.c
     ${PROJECT_SOURCE_DIR}/machine_pin.c
     ${PROJECT_SOURCE_DIR}/machine_pwm.c
+    ${PROJECT_SOURCE_DIR}/machine_rtc.c
     ${PROJECT_SOURCE_DIR}/machine_spi.c
     ${PROJECT_SOURCE_DIR}/machine_timer.c
     ${PROJECT_SOURCE_DIR}/machine_uart.c
@@ -154,6 +156,7 @@ set(PICO_SDK_COMPONENTS
     pico_sync
     pico_time
     pico_unique_id
+    pico_util
     tinyusb_common
     tinyusb_device
 )