plat/arm: css: Convert CSS_SYSTEM_PWR_STATE from macro to inline function

This is to fix a number of Coverity Scan DEADCODE defects, CID numbers
listed below, as reported from
https://scan.coverity.com/projects/arm-software-arm-trusted-firmware

CID 267023
CID 267022
CID 267020

Change-Id: I2963a799b210149e84ccab5c5b9082267ddfe337
Signed-off-by: Nariman Poushin <nariman.poushin@linaro.org>
diff --git a/plat/arm/css/drivers/scp/css_pm_scpi.c b/plat/arm/css/drivers/scp/css_pm_scpi.c
index 23fd80e..18e71f6 100644
--- a/plat/arm/css/drivers/scp/css_pm_scpi.c
+++ b/plat/arm/css/drivers/scp/css_pm_scpi.c
@@ -25,7 +25,7 @@
 	uint32_t system_state = scpi_power_on;
 
 	/* Check if power down at system power domain level is requested */
-	if (CSS_SYSTEM_PWR_STATE(target_state) == ARM_LOCAL_STATE_OFF)
+	if (css_system_pwr_state(target_state) == ARM_LOCAL_STATE_OFF)
 		system_state = scpi_power_retention;
 
 	/* Cluster is to be turned off, so disable coherency */