refactor(stm32mp1): move PIE flag to SP_min

The PIE compilation is used only for BL32, move the ENABLE_PIE to
sp_min-stm32mp1.mk file. Override PIE flags, as sp_min.mk file is
included after the flags are set in Makefile.
The BL2_IN_XIP_MEM was added for a feature not yet upstreamed.
It is then removed from platform.mk file.

Change-Id: If055e51e0f160f99cd4e4cf68ca718d4d693119c
Signed-off-by: Yann Gautier <yann.gautier@st.com>
Signed-off-by: Nicolas Toromanoff <nicolas.toromanoff@foss.st.com>