refactor(st): move GIC code to common directory

The GIC v2 initialization code could be shared to other ST platforms.
The stm32mp1_gic.c file is then moved to common directory, and renamed
stm32mp_gic.c.
The functions are also prefixed with stm32mp_gic.

Change-Id: I60820823b470217d3a95cc569f941c2cb923dfa9
Signed-off-by: Yann Gautier <yann.gautier@st.com>
diff --git a/plat/st/common/include/stm32mp_common.h b/plat/st/common/include/stm32mp_common.h
index 54cfcfb..1d592bd 100644
--- a/plat/st/common/include/stm32mp_common.h
+++ b/plat/st/common/include/stm32mp_common.h
@@ -35,6 +35,9 @@
 /* Return the base address of the RCC peripheral */
 uintptr_t stm32mp_rcc_base(void);
 
+void stm32mp_gic_pcpu_init(void);
+void stm32mp_gic_init(void);
+
 /* Check MMU status to allow spinlock use */
 bool stm32mp_lock_available(void);