mfd: Support configurable numbers of DCDCs and ISINKs on WM8350

Some WM8350 variants have fewer DCDCs and ISINKs. Identify these at
probe and refuse to use the absent DCDCs when running on these chips.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@openedhand.com>