boards: seeed: adds the Seeed Studio XIAO RP2350

Adds basic support for the "Seeed Studio XIAO RP2350" that comes with
2MB on-board QSPI Flash, a yellow user LED on GP25, UART0 as console,
I2C1 and SPI0 as primjary default serial busses, UART1, I2C0 and SPI1
as secondary serial busses, free access to ADC0 - ADC2, and the
following add-ons:

- support for the digital on-board RGB LED at GP22 over PIO0
  with power switch (enable) on GP23 (as fixed regulator)
- support for the on-board battery charging circuit with simple
  voltage monitoring of VBAT/2 at ADC3 and gated over a second
  power switch on GP19 (as fixed regulator)
- support for the Seeed Studio XIAO pin header
- support for the Seeed Studio XIAO expantion test pads with
  additional signals (secondary busses) on the PCB bottom side

Signed-off-by: Stephan Linz <linz@li-pro.net>