ARM: stm32: Make ECDSA authentication available to U-Boot

With U-Boot having access to ROM API call table, it is possible to use
the ROM API call it authenticate e.g. signed kernel fitImages using the
BootROM ECDSA support. Make this available by pulling the ECDSA BootROM
call support from SPL-only guard.

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>