drivers: i2c_nrfx_twi: Make use of NRFX_TWI_FLAG_SUSPEND

Make use of the new flag introduced in nrfx v1.7.2 that suspends
transfers, in order to properly handle scattered I2C transactions,
especially to perform i2c_burst_write() in the way expected by
display drivers, i.e. as a continuous transaction on the bus,
without the repeated START between its two parts.

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