drivers: adc: Remove channel_count property for Renesas driver

Currently, in Renesas adc driver, channel_count is used
as the maximum index of the channels can be supported.
However, the value input in dts of "channel_count"
represents the total number of supported channels.

After consideration, we have decided to remove this
reduntant property.

Signed-off-by: Khoa Nguyen <khoa.nguyen.xh@renesas.com>