boards: nucleo_wb55rg: Update regarding supported M0 BLE f/w

Since STM32WBCube release V1.13.2, only "HCI Only" f/w are now compatible
for a use with Zephyr.
"Full stack" f/w which used to be supported are not compatible anymore.

Update board documentation to mention this restriction.

Additionally, update flash partition to reflect the gain implied
by the use of this smaller f/w.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>