radio: RTC6226: implement file read for rtc6226 driver

FM app is using the VIDIOC DQBUF ioctl to read the
event from rtc6226 driver. VIDIOC DQBUF ioctl needs
the v4l2buf type as V4L2_BUF_TYPE_PRIVATE and it
is Deprecated and not recommend to use in latest
kernel version.Also upstream kernel code doesn't
have the VIDIOC DQBUF as a valid ioctl. 
Implement file read functionality to read the events
from rtc6226 driver.

CRs-Fixed: 2702667
Change-Id: I747411e362e319ff8f45e9005e0c677eeef854dc
Signed-off-by: himta ram <hram@codeaurora.org>