Signed-off-by: Frederic Pillon <frederic.pillon@st.com>

Signed-off-by: Frederic Pillon <frederic.pillon@st.com>

Signed-off-by: Peter Lawrence <12226419+majbthrd@users.noreply.github.com>
Co-authored-by: Frederic Pillon <frederic.pillon@st.com>

This turns SPISettings into a "literal type" and allows variables using
it to be constexpr to guarantee the constructor is executed at
compiletime. This requires (with C++11) that all variables are set using
initializers instead of assignments and that the regular if cascade is
replaced by a ternary if. It also requires explicit initializers for all
values (omitted variables were previously initialized to zero anyway).

This allows accessing some of the signals internally connected to the
SubGhz radio (that would have been GPIO signals if the radio was
external). This also allocates and exposes the SPI object connected to
the SubGhz radio block and handles attaching handlers to the radio
interrupt.

Note that the DIO signals are *not* exposed, since there is no way to
read them directly (and indirectly reading them through the IRQ pending
flag does not work in all cases).