clk: add accuracy support for fixed clock

This patch adds support for accuracy retrieval on fixed clocks.
It also adds a new dt property called 'clock-accuracy' to define the clock
accuracy.

This can be usefull for oscillator (RC, crystal, ...) definitions which are
always given an accuracy characteristic.

Signed-off-by: Boris BREZILLON <b.brezillon@overkiz.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>

diff --cc Documentation/devicetree/bindings/clock/fixed-clock.txt
Simple merge

diff --cc drivers/clk/clk-fixed-rate.c
Simple merge

diff --cc include/linux/clk-provider.h
Simple merge