rp2: Use 0=Monday datetime convention in RTC.

The RTC in rp2 can store any, even wrong, number as a weekday in RTC.  It
was, however, discussed in #7394 that we would like to unify all ports and
use 0 as Monday, not Sunday in the machine.RTC implementation.

This patch makes sure that the default date set in RTC is adheres to this
convention.  It also fixes the example in quickref to use proper weekday to
avoid confusion.
Signed-off-by: Krzysztof Adamski <k@japko.eu>

docs/rp2/quickref.rst

@@ -202,7 +202,7 @@ See :ref:`machine.RTC <machine.RTC>` ::
     from machine import RTC
 
     rtc = RTC()
-    rtc.datetime((2017, 8, 23, 1, 12, 48, 0, 0)) # set a specific date and time
+    rtc.datetime((2017, 8, 23, 2, 12, 48, 0, 0)) # set a specific date and time
     rtc.datetime() # get date and time
 
 WDT (Watchdog timer)

ports/rp2/main.c

@@ -80,7 +80,7 @@ int main(int argc, char **argv) {
         .year = 2021,
         .month = 1,
         .day = 1,
-        .dotw = 5, // 0 is Sunday, so 5 is Friday
+        .dotw = 4, // 0 is Monday, so 4 is Friday
         .hour = 0,
         .min = 0,
         .sec = 0,