regulator: ab8500: Split up probe() into manageable pieces

ab8500's probe() function is becoming quite large, so in the lead
up to Device Tree enablement which will fork the thread of execution
this patch splits it into 3 main areas; basic error checking will
remain in probe(), but regulator register initialisation and regulator
registration have been moved to their own functions which will
be called in sequence by probe() and the DT equivalent.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>