samples: drivers: jesd216 density expressed in bits

In the sample, print the jesd216_bfp_density is expressed
in bits. add the unit to avoid confusion with the Flash
density expressed in bytes (dw2 entry)
Page size is for programming expressed in bytes, meaning
program execution on bytes basis or here page basis.

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