Refactor fvp gic code to be a generic driver

Refactor the FVP gic code in plat/fvp/fvp_gic.c to be a generic ARM
GIC driver in drivers/arm/gic/arm_gic.c. Provide the platform
specific inputs in the arm_gic_setup() function so that the driver
has no explicit dependency on platform code.

Provide weak implementations of the platform interrupt controller
API in a new file, plat/common/plat_gic.c. These simply call through
to the ARM GIC driver.

Move the only remaining FVP GIC function, fvp_gic_init() to
plat/fvp/aarch64/fvp_common.c and remove plat/fvp/fvp_gic.c

Fixes ARM-software/tf-issues#182

Change-Id: Iea82fe095fad62dd33ba9efbddd48c57717edd21
diff --git a/include/drivers/arm/gic_v2.h b/include/drivers/arm/gic_v2.h
index 1859a8e..4c6b0dc 100644
--- a/include/drivers/arm/gic_v2.h
+++ b/include/drivers/arm/gic_v2.h
@@ -36,6 +36,10 @@
 #define MAX_PPIS		14
 #define MAX_SGIS		16
 
+#define MIN_SGI_ID		0
+#define MIN_PPI_ID		16
+#define MIN_SPI_ID		32
+
 #define GRP0			0
 #define GRP1			1
 #define GIC_PRI_MASK		0xff