boards: arm: nucleo_wb55rg enables hsi48 for Bluetooth

The nucleo_wb55rg platform has Bluetooth feature that
requires the HSI48.
The board DTS enables the HSI48MHz clock for that purpose.
And also selects it as the source (default) of the CLK48.
Note that the clk48 node must be enabled for any another source.

Signed-off-by: Francois Ramu <francois.ramu@st.com>