drivers: i2c_nrfx_twim: Use concatenation buffer by default

Issue an error logging message when the i2c_nrfx_twim driver lacks
a concatenation buffer big enough to properly handle a call to
i2c_burst_write() function, to give the user a hint what is wrong.

Also use by default a 16-bytes long concatenation buffer for every
instance of the i2c_nrfx_twim driver. Such value should cover most
of the simple uses of the i2c_burst_write() function, like those
in the stmemsc sensor drivers, and when a longer buffer is needed,
the user will be provided with the above message pointing to the
property that should be adjusted.

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