pinctrl: sh-pfc: r8a7796: Deduplicate VIN5 pin definitions

Use union vin_data16 and VIN_DATA_PIN_GROUP() to reduce redundancies in
pin definitions for the VIN5 channel.

This reduces kernel size by 240 bytes.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>