rp2040: Fix I2C frequency for all boards

The DW driver used by the RP2040 boards uses a Kconfig symbol
to determine the clock speed of the board. This value was
set incorrectly by the pico board to 100MHz (instead of 125).
For the other boards, it wasn't set at all.
This commit fixes the kconfig defaults. In the future, this
value should be taken directly from the devicetree. This is
not currently possible as there is no clock control driver
for the RP2040 (see #52901).

Signed-off-by: Yonatan Schachter <yonatan.schachter@gmail.com>