clk: si5351: Add PLL soft reset

This is according to figure 12 ("I2C Programming Procedure") in
"Si5351A/B/C Data Sheet"
(https://www.silabs.com/Support%20Documents/TechnicalDocs/Si5351-B.pdf).

Without the PLL soft reset, we were unable to get three outputs
working at the same time.

According to Silicon Labs support, performing PLL soft reset will only
be noticeable if the PLL parameters have been changed.

Signed-off-by: Jacob Siverskog <jacob@teenage.engineering>
Signed-off-by: Jens Rudberg <jens@teenage.engineering>
Acked-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>