clk: Add driver for the si544 clock generator chip

This patch adds the driver and devicetree documentation for the
Silicon Labs SI544 clock generator chip. This is an I2C controlled
oscillator capable of generating clock signals ranging from 200kHz
to 1500MHz.

Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>
[sboyd: assign max_freq to 0 in is_valid_frequency() to squelch warning]
Signed-off-by: Stephen Boyd <sboyd@kernel.org>