board: rockchip: add Radxa ROCK5B Rk3588 board

ROCK 5B is a Rockchip RK3588 based SBC (Single Board Computer) by Radxa.

There are tree variants depending on the DRAM size : 4G, 8G and 16G.

Specification:

    Rockchip Rk3588 SoC
    4x ARM Cortex-A76, 4x ARM Cortex-A55
    4/8/16GB memory LPDDR4x
    Mali G610MC4 GPU
    MIPI CSI 2 multiple lanes connector
    eMMC module connector
    uSD slot (up to 128GB)
    2x USB 2.0, 2x USB 3.0
    2x HDMI output, 1x HDMI input
    Ethernet port
    40-pin IO header including UART, SPI, I2C and 5V DC power in
    USB PD over USB Type-C
    Size: 85mm x 54mm

Kernel commits:
a1d3281450ab ("arm64: dts: rockchip: Add rock-5b board")
6fb13f888f2a ("arm64: dts: rockchip: Update sdhci alias for rock-5b")

Signed-off-by: Eugen Hristev <eugen.hristev@collabora.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
diff --git a/arch/arm/dts/rk3588-rock-5b-u-boot.dtsi b/arch/arm/dts/rk3588-rock-5b-u-boot.dtsi
new file mode 100644
index 0000000..2386edf
--- /dev/null
+++ b/arch/arm/dts/rk3588-rock-5b-u-boot.dtsi
@@ -0,0 +1,22 @@
+// SPDX-License-Identifier: (GPL-2.0+ OR MIT)
+/*
+ * Copyright (c) 2023 Collabora Ltd.
+ */
+
+#include "rk3588-u-boot.dtsi"
+
+/ {
+	aliases {
+		mmc0 = &sdmmc;
+	};
+
+	chosen {
+		u-boot,spl-boot-order = &sdmmc;
+	};
+};
+
+&sdmmc {
+	bus-width = <4>;
+	u-boot,dm-spl;
+	status = "okay";
+};