mips: lantiq: add support for clk_get_parent()

Provide a simple implementation of clk_get_parent() in the
lantiq subarch so that callers of it will build without errors.

Fixes this build error:
ERROR: modpost: "clk_get_parent" [drivers/iio/adc/ingenic-adc.ko] undefined!

Fixes: 171bb2f1 ("MIPS: Lantiq: Add initial support for Lantiq SoCs")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Suggested-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Cc: linux-mips@vger.kernel.org
Cc: John Crispin <john@phrozen.org>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: Jonathan Cameron <jic23@kernel.org>
Cc: linux-iio@vger.kernel.org
Cc: Russell King <linux@armlinux.org.uk>
Cc: Andy Shevchenko <andy.shevchenko@gmail.com>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Acked-by: John Crispin <john@phrozen.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>