boards: arm: adafruit_kb2040: Fix missing flash controller
Fixes an issue whereby the flash controller for this board was
missing.
chosen {
zephyr,sram = &sram0;
zephyr,flash = &flash0;
+ zephyr,flash-controller = &ssi;
zephyr,console = &uart0;
zephyr,shell-uart = &uart0;
zephyr,code-partition = &code_partition;
