clk: Add Si5341/Si5340 driver

Adds a driver for the Si5341 and Si5340 chips. The driver does not fully
support all features of these chips, but allows the chip to be used
without any support from the "clockbuilder pro" software.

If the chip is preprogrammed, that is, you bought one with some defaults
burned in, or you programmed the NVM in some way, the driver will just
take over the current settings and only change them on demand. Otherwise
the input must be a fixed XTAL in its most basic configuration (no
predividers, no feedback, etc.).

The driver supports dynamic changes of multisynth, output dividers and
enabling or powering down outputs and multisynths.

Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>
[sboyd@kernel.org: Mark some things static, use BIT_ULL for big bits and
ULL for big constants]
Signed-off-by: Stephen Boyd <sboyd@kernel.org>