boards: sparkfun: adds the RP2040 mikroBUS development board

Adds basic support for the "Sparkfun RP2040 mikroBUS Development
Board" that comes with 16MB on-board QSPI Flash, a blue user LED
on GP25, UART0 as console, I2C1 and SPI0 as default serial busses,
free access to ADC0 - ADC3, and the following add-ons:

- support for the digital on-board RGB LED at GP8 over PIO0
- support for the on-board battery fuel gauge sensor over I2C1
  that monitors the single cell battery charging circuit
- support for the on-board microSD card as SDHC over SPI1 at 12MHz
- support for the on-board Qwiic or STEMMA QT connector
- support for the on-board mikroBUS socket
- support for the Thing Plus or Feather pin header

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