rp2/main: Set the default clock frequency at boot.

As a side effect, the peripheral clock will be set to 48Mhz and both UART
and I2C will not be affected by CPu speed changed using `machine.freq()`.

With the change the UART baud rate range is 50 to 3_000_000.
Signed-off-by: robert-hh <robert@hammelrath.com>

ports/rp2/main.c

@@ -80,6 +80,9 @@ int main(int argc, char **argv) {
     pendsv_init();
     soft_timer_init();
 
+    // Set the MCU frequency and as a side effect the peripheral clock to 48 MHz.
+    set_sys_clock_khz(125000, false);
+
     #if MICROPY_HW_ENABLE_UART_REPL
     bi_decl(bi_program_feature("UART REPL"))
     setup_default_uart();
@@ -231,6 +234,10 @@ int main(int argc, char **argv) {
 
         gc_sweep_all();
         mp_deinit();
+        #if MICROPY_HW_ENABLE_UART_REPL
+        setup_default_uart();
+        mp_uart_init();
+        #endif
     }
 
     return 0;