clk, clk-si5341: Support multiple input ports

The Si5341 and Si5340 have multiple input clock options. So far, the driver
only supported the XTAL input, this adds support for the three external
clock inputs as well.

If the clock chip isn't programmed at boot, the driver will default to the
XTAL input as before. If there is no "xtal" clock input available, it will
pick the first connected input (e.g. "in0") as the input clock for the PLL.
One can use clock-assigned-parents to select a particular clock as input.

Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>
Link: https://lkml.kernel.org/r/20200107075340.14528-1-mike.looijmans@topic.nl
Signed-off-by: Stephen Boyd <sboyd@kernel.org>