Add driver for the si514 clock generator chip

This patch adds the driver and devicetree documentation for the
Silicon Labs SI514 clock generator chip. This is an I2C controlled
oscillator capable of generating clock signals ranging from 100kHz
to 250MHz.

Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>
[sboyd@codeaurora.org: Drop clk.h include, remove some casts]
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>