firmware: scmi: use scmi_proto_driver_get() function to get SCMI protocol driver

If there is a SoC specific SCMI protocol driver, using
scmi_proto_driver_get() function can avoid to add SoC specific code to
scmi_agent-uclass.c.

Signed-off-by: Alice Guo <alice.guo@nxp.com>
diff --git a/drivers/power/regulator/scmi_regulator.c b/drivers/power/regulator/scmi_regulator.c
index 79db1a6..7d2db1e 100644
--- a/drivers/power/regulator/scmi_regulator.c
+++ b/drivers/power/regulator/scmi_regulator.c
@@ -8,6 +8,7 @@
 #include <dm.h>
 #include <errno.h>
 #include <scmi_agent.h>
+#include <scmi_agent-uclass.h>
 #include <scmi_protocols.h>
 #include <asm/types.h>
 #include <dm/device.h>
@@ -202,3 +203,10 @@
 	.id = UCLASS_NOP,
 	.bind = scmi_regulator_bind,
 };
+
+static struct scmi_proto_match match[] = {
+	{ .proto_id = SCMI_PROTOCOL_ID_VOLTAGE_DOMAIN },
+	{ /* Sentinel */ }
+};
+
+U_BOOT_SCMI_PROTO_DRIVER(scmi_voltage_domain, match);