drivers: i2c_nrfx_twim: Add handling of buffers located in flash

TWIM peripherals cannot perform write transactions from buffers
located in flash. The content of such buffers needs to be copied
to RAM before the actual transfer can be requested.
This commits adds a new property (zephyr,flash-buf-max-size) that
informs the driver how much space in RAM needs to be reserved for
such copying and adds proper handling of buffers located in flash.
This fixes an issue that caused that e.g. the DPS310 sensor driver
did not work on nRF SoCs that only have TWIM, not TWI peripherals.

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