configs: stm32mp1: replace STM32MP1_TRUSTED by TFABOOT

Activate ARCH_SUPPORT_TFABOOT and replace the arch stm32mp
specific config CONFIG_STM32MP1_TRUSTED by the generic CONFIG_TFABOOT
introduced by the commit 535d76a1 ("armv8: layerscape: Add TFABOOT
support").
This config CONFIG_TFABOOT is activated for the trusted boot chain,
when U-Boot is loaded by TF-A.

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