stm32mp1: 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
stm32_gic.c.
The functions are also aligned with the already defined function
stm32_gic_enable_spi in common code, i.e. prefix is stm32_gic.

Change-Id: I60820823b470217d3a95cc569f941c2cb923dfa9
Signed-off-by: Yann Gautier <yann.gautier@st.com>
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/140376


Reviewed-by: CITOOLS <smet-aci-reviews@lists.codex.cro.st.com>