mpc85xx: add SPI kernel loader for TP-Link TL-WDR4900 v1

Similar to the lzma-loader on our MIPS targets, the spi-loader acts as
a second-stage loader that will then load and start the actual kernel.
As the TL-WDR4900 uses SPI-NOR and the P1010 family does not have support
for memory mapping of this type of flash, this loader needs to contain a
basic driver for the FSL ESPI controller.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>
(cherry picked from commit a296055b82)

target/linux/mpc85xx/image/spi-loader/include/serial.h

@@ -0,0 +1,9 @@
+// SPDX-License-Identifier: BSD-2-Clause
+
+#pragma once
+
+int serial_console_getchar(void);
+int serial_console_tstc(void);
+void serial_console_putchar(char c);
+void serial_console_init(void);
+