hwmon: epm_adc: Register epm_adc with misc

epm_adc is required by misc to access the file I/O that is
exposed by the epm_adc interface for clients to issue
requests to read the ADC.

Change-Id: Ib731c5dc5f6844d28f9031756ab2735b141f8c39
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>