Disallow setting of overvoltage via pcf50606 kernel driver

[laforge@…]

currently there are no kernel-enforced limits for the voltage regulator
settings. This means that [root] user can set overvoltages on various parts of
the system.

We need something like a table of maximum voltages (for each system revision).

Ideally this should become part of a platform_device private data structure,
something like (per-regulator):

struct {

u_int16_t max_voltage;
u_int16_t min_voltage;
unsigned int flags; /* such as F_DEFAULT_ON */
char *name;

};

the voltage setting functions should then not only respect the limits by
pcf50606 hardware, but the limits present in that structure.

[laforge@…]

overvoltage limiting has been implemented as part of recent pcf50606 cleanup
(for hxd8).