[media] media: rc: nuvoton: switch attribute wakeup_data to text

Switch attribute wakeup_data from binary to a text attribute.
This makes it easier to handle in userspace and allows to
use the output of tools like mode2 almost as is to set a
wakeup sequence.
Changing to a text format and values in microseconds also
makes the userspace interface independent of the setting of
SAMPLE_PERIOD in the driver.

In addition document the new sysfs attribute in
Documentation/ABI/testing/sysfs-class-rc-nuvoton.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>