mips: bcm63xx: add support for clk_get_parent()

BCM63XX selects HAVE_LEGACY_CLK but does not provide/support
clk_get_parent(), so add a simple implementation of that
function so that callers of it will build without errors.

Fixes these build errors:

mips-linux-ld: drivers/iio/adc/ingenic-adc.o: in function `jz4770_adc_init_clk_div':
ingenic-adc.c:(.text+0xe4): undefined reference to `clk_get_parent'
mips-linux-ld: drivers/iio/adc/ingenic-adc.o: in function `jz4725b_adc_init_clk_div':
ingenic-adc.c:(.text+0x1b8): undefined reference to `clk_get_parent'

Fixes: e7300d04 ("MIPS: BCM63xx: Add support for the Broadcom BCM63xx family of SOCs." )
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: kernel test robot <lkp@intel.com>
Suggested-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Cc: Artur Rojek <contact@artur-rojek.eu>
Cc: Paul Cercueil <paul@crapouillou.net>
Cc: linux-mips@vger.kernel.org
Cc: Jonathan Cameron <jic23@kernel.org>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Cc: linux-iio@vger.kernel.org
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Russell King <linux@armlinux.org.uk>
Cc: bcm-kernel-feedback-list@broadcom.com
Cc: Jonas Gorski <jonas.gorski@gmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Acked-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>