stm32mp: add syscon for STGEN

Add STGEN as SYSCON device: allow access to device address
defined in device tree

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
diff --git a/arch/arm/mach-stm32mp/Makefile b/arch/arm/mach-stm32mp/Makefile
index 4620869..a495c53 100644
--- a/arch/arm/mach-stm32mp/Makefile
+++ b/arch/arm/mach-stm32mp/Makefile
@@ -6,5 +6,6 @@
 
 obj-y += cpu.o
 obj-y += dram_init.o
+obj-y += syscon.o
 
 obj-$(CONFIG_SPL_BUILD) += spl.o