add function to set clock rate

Reference: /n/atom/patch/applied/pisetclk
Date: Sun Jan  3 19:38:58 CET 2016
Signed-off-by: quanstro@quanstro.net

--- /sys/src/9/bcm/vcore.c	Sun Jan  3 19:38:49 2016
+++ /sys/src/9/bcm/vcore.c	Sun Jan  3 19:38:50 2016
@@ -40,6 +40,8 @@
 	TagSetpower	= 0x00028001,
 		Powerwait	= 1<<1,
 	TagGetclkspd= 0x00030002,
+	TagGetclkmax= 0x00030004,
+	TagSetclkspd= 0x00038002,
 	TagGettemp	= 0x00030006,
 	TagFballoc	= 0x00040001,
 	TagFbfree	= 0x00048001,
@@ -306,6 +308,22 @@
 	if(vcreq(TagGetclkspd, buf, sizeof(buf[0]), sizeof(buf)) != sizeof buf)
 		return 0;
 	return buf[1];
+}
+
+/*
+ * Set clock rate to hz (or max speed if hz == 0)
+ */
+void
+setclkrate(int clkid, ulong hz)
+{
+	u32int buf[2];
+
+	buf[0] = clkid;
+	if(hz != 0)
+		buf[1] = hz;
+	else if(vcreq(TagGetclkmax, buf, sizeof(buf[0]), sizeof buf) != sizeof buf)
+		return;
+	vcreq(TagSetclkspd, buf, sizeof(buf), sizeof(buf));
 }
 
 /*