stm32mp1: define STM32MP15C-ED1

STM32MP15C-ED1 is the child board plugged on EV1 mother board. ED1 can
be used without its mother board with some interface restrictions
(I.e. no USB-C). This change defines the platform flavor. The several
components (optee_os, tf-a, u-boot, linux) already support ED1 DTS file
identifier.

Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>
Signed-off-by: Etienne Carriere <etienne.carriere@st.com>
Acked-by: Jerome Forissier <jerome@forissier.org>