regmap: Add precious registers to the driver interface

Some devices are sensitive to reads on their registers, especially for
things like clear on read interrupt status registers. Avoid creating
problems with these with things like debugfs by allowing drivers to tell
the core about them. If a register is marked as precious then the core
will not internally generate any reads of it.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>