extres/regulators: Add sysctls for regulators

For each regulators create an hw.regulator.<regname>. :
uvolt: Current value
always_on: 1 If the reg is always on
boot_on: 1 If the reg is set at boot time
enable_cnt: Number of consumer(s)
enable_delay: Delay before enabling the regulator
ramp_delay: The Ramp delay
max_uamp: The maximum value of the regulator in uAmps
min_uamp: The minimal value of the regulator in uAmps
max_uvolt: The maximum value of the regulator in uVolts
min_uvolt: The minimal value of the regulator in uVolts

Reviewed by: ian
Differential Revision: https://reviews.freebsd.org/D14578