soc: stm32wb: Set BT_USER_PHY_UPDATE as supported

STM32WB Controller supports application to initiate the "PHY Update
Procedure" (BT_USER_PHY_UPDATE) while it doesn't support it to be
automatically triggered on connection establishment (BT_AUTO_PHY_UPDATE).

Default BT_USER_PHY_UPDATE to true, which automatically defaults
BT_AUTO_PHY_UPDATE to false.

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