arm64: dts: renesas: r8a77995: don't use deprecated renesas,gpio-rcar compat string

The compat string renesas,gpio-rcar has been deprecated since v4.14,
the same release that r8a77990 SoC support was added. Thus
renesas,gpio-rcar can safely be removed without any risk of behaviour
changes between old and new mainline kernels and DTBs.

Signed-off-by: Simon Horman <horms+renesas@verge.net.au>