arm: mvebu: Add armada-xp-gp-u-boot.dtsi for U-Boot properties

Add some missing "u-boot,dm-pre-reloc;" properties to UART0, SPI
controller and SPI NOR flash node to enable usage in SPL. Otherwise
these devices will not be available.
Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Dennis Gilmore <dgilmore@redhat.com>
Tested-by: Dennis Gilmore <dgilmore@redhat.com>

arch/arm/dts/armada-xp-gp-u-boot.dtsi

+// SPDX-License-Identifier: GPL-2.0+
+
+/ {
+	soc {
+		internal-regs {
+			serial@12000 {
+				u-boot,dm-pre-reloc;
+			};
+		};
+	};
+};
+
+&spi0 {
+	u-boot,dm-pre-reloc;
+
+	spi-flash@0 {
+		u-boot,dm-pre-reloc;
+	};
+};