clk: meson: add clk-input helper function

clk: meson: add clk-input helper function

Add the clock input helper function. Several amlogic clock controllers
will now be registering bypass clock input. Instead of copying this
code in every of them, let's make an helper function for it

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
[narmstrong: fixed up to apply on Makefile and clkc.h]
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://lkml.kernel.org/r/20181204165819.21541-2-jbrunet@baylibre.com