mtd: spi: renesas: Add R-Car Gen4 DT compatible

Add device tree compatible string "renesas,rcar-gen4-rpc-if" to
the driver to match on upstream RPC DT node in R-Car Gen4 DTs.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
diff --git a/drivers/spi/renesas_rpc_spi.c b/drivers/spi/renesas_rpc_spi.c
index 9aab71d..f1e6f9f 100644
--- a/drivers/spi/renesas_rpc_spi.c
+++ b/drivers/spi/renesas_rpc_spi.c
@@ -465,6 +465,7 @@
 static const struct udevice_id rpc_spi_ids[] = {
 	{ .compatible = "renesas,r7s72100-rpc-if" },
 	{ .compatible = "renesas,rcar-gen3-rpc-if" },
+	{ .compatible = "renesas,rcar-gen4-rpc-if" },
 	{ }
 };