ARM: stm32: Fix ECDSA authentication with Dcache enabled

In case Dcache is enabled while the ECDSA authentication function is
called via BootROM ROM API, the MMU tables are set up and the BootROM
region is not marked as executable, so an attempt to run code from it
results in a hang. Mark the BootROM region as executable as suggested
by Patrick to prevent the hang.

Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Signed-off-by: Marek Vasut <marex@denx.de>