MIPS: ralink: define stubs for clk_set_parent to fix compile testing

The Ralink MIPS platform does not use Common Clock Framework and does
not define certain clock operations leading to compile test failures:

    /usr/bin/mips-linux-gnu-ld: drivers/usb/phy/phy-tegra-usb.o: in function `tegra_usb_phy_init':
    phy-tegra-usb.c:(.text+0x1dd4): undefined reference to `clk_get_parent'

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Acked-by: John Crispin <john@phrozen.org>
Reviewed-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/ralink/clk.c

@@ -70,6 +70,20 @@ long clk_round_rate(struct clk *clk, unsigned long rate)
 }
 EXPORT_SYMBOL_GPL(clk_round_rate);
 
+int clk_set_parent(struct clk *clk, struct clk *parent)
+{
+	WARN_ON(clk);
+	return -1;
+}
+EXPORT_SYMBOL_GPL(clk_set_parent);
+
+struct clk *clk_get_parent(struct clk *clk)
+{
+	WARN_ON(clk);
+	return NULL;
+}
+EXPORT_SYMBOL_GPL(clk_get_parent);
+
 void __init plat_time_init(void)
 {
 	struct clk *clk;