Fix MISRA Rule 5.7 Part 2

Follow convention of shorter names for smaller scope to fix
violations of MISRA rule 5.7

To prevent violation of directive 4.5 having variable name channel
in css_pm_scmi.c not being typographically ambiguous change macro
argument CHANNEL in css_mhu_doorbell.h change argument to _channel
to fit with our convention which is a permitted exception of
directive 4.5 for this project

Rule 5.7: A tag name shall be a unique identifier

Fixed for:
    make LOG_LEVEL=50 PLAT=juno

Change-Id: I147cdb13553e83ed7df19149b282706db115d612
Signed-off-by: Daniel Boulby <daniel.boulby@arm.com>

plat/arm/css/drivers/mhu/css_mhu_doorbell.h

@@ -18,8 +18,8 @@
 #define MHU_V2_ACCESS_REQ_OFFSET		0xF88
 #define MHU_V2_ACCESS_READY_OFFSET		0xF8C
 
-#define SENDER_REG_STAT(CHANNEL)	(0x20 * (CHANNEL))
-#define SENDER_REG_SET(CHANNEL)		(0x20 * (CHANNEL)) + 0xC
+#define SENDER_REG_STAT(_channel)	(0x20 * (_channel))
+#define SENDER_REG_SET(_channel)	((0x20 * (_channel)) + 0xC)
 
 /* Helper macro to ring doorbell */
 #define MHU_RING_DOORBELL(addr, modify_mask, preserve_mask)	do {	\

plat/arm/css/drivers/scp/css_pm_scmi.c

@@ -69,7 +69,7 @@ typedef enum {
 static void *scmi_handle;
 
 /* The SCMI channel global object */
-static scmi_channel_t scmi_channel;
+static scmi_channel_t channel;
 
 ARM_INSTANTIATE_LOCK;
 
@@ -308,9 +308,9 @@ scmi_channel_plat_info_t plat_css_scmi_plat_info = {
 
 void plat_arm_pwrc_setup(void)
 {
-	scmi_channel.info = &plat_css_scmi_plat_info;
-	scmi_channel.lock = ARM_LOCK_GET_INSTANCE;
-	scmi_handle = scmi_init(&scmi_channel);
+	channel.info = &plat_css_scmi_plat_info;
+	channel.lock = ARM_LOCK_GET_INSTANCE;
+	scmi_handle = scmi_init(&channel);
 	if (scmi_handle == NULL) {
 		ERROR("SCMI Initialization failed\n");
 		panic();