arm: stm32mp: protect DBGMCU_IDC access with BSEC

As debugger must be totally closed on Sec closed chip,
the DBGMCU_IDC register is no more accessible (self
hosted debug is disabled with OTP).

This patch adds a function bsec_dbgswenable() to check
if the DBGMCU registers are available before to access them:
BSEC_DENABLE.DBGSWENABLE = self hosted debug status.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>