pinctrl: sh-pfc: r8a77990: GP6_9 does not have pull-down capability

Hence remove the SH_PFC_PIN_CFG_PULL_DOWN flag from the GP6_9 GPIO
description.

Fixes: 83f6941a ("pinctrl: sh-pfc: r8a77990: Add bias pinconf support")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>