MAINTAINERS: Split "STM32 platforms"

Add a new "STM32 Wireless platforms" area split from existing
"STM32 platforms".
STM32 Wireless covers STM32 BLE areas:
- STM32WB* based ST boards
- STM32WB* drivers/bluetooth/hci drivers
- STM32WB* soc/st/stm32

Additionally, the following domains are added to STM32 Wireless platforms:
- BlueNRG drivers/bluetooth/hci drivers
- shield x_nucleo_bnrg2a1, x_nucleo_idb05a1 and x_nucleo_wb05kn1

Signed-off-by: Erwan Gouriou <erwan.gouriou@st.com>