pinctrl: sh-pfc: r8a77970: remove SH_PFC_PIN_CFG_DRIVE_STRENGTH flag

The datasheet does not document any registers to control drive strength,
and no drive strength registers are for this reason described for this
SoC. The flags indicating that drive strength can be controlled are
however set for some pins in the driver.

This leads to a NULL pointer dereference when the sh-pfc core tries to
access the struct describing the drive strength registers, for example
when reading the sysfs file pinconf-pins.

Fix this by removing the SH_PFC_PIN_CFG_DRIVE_STRENGTH from all pins.

Fixes: b92ac66a ("pinctrl: sh-pfc: Add R8A77970 PFC support")
Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>