drivers: i2c_nrfx_twim: Fix frequency configuration

Change the way this shim driver configures the I2C frequency, so that
it is possible to use also 1 MHz on nRF5340 (the nrfx driver performs
extra initialization steps for this frequency, hence it needs to be
reinitialized when the shim is reconfigured).
Correct the shim to handle selection of 1 MHz (or FAST_PLUS) bitrate
both through dts and I2C API.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>