mpc5xxx: Add stub implementation of cache functions

Some drivers (e.g. net/e1000) reference these functions. So, this
fixes the build of MVBC_P board.

I'm not familiar with the MPC5xxx platform, maybe a full
implementation shall be implemented instead of this stub in the
future.

Signed-off-by: Vasili Galka <vvv444@gmail.com>
Cc: Wolfgang Denk <wd@denx.de>, Marek Vasut <marex@denx.de>
Acked-by: Wolfgang Denk <wd@denx.de>

arch/powerpc/cpu/mpc5xxx/Makefile

@@ -7,6 +7,7 @@
 
 extra-y	= start.o
 extra-y += traps.o
+obj-y  += cache.o
 obj-y  += io.o
 obj-y  += firmware_sc_task_bestcomm.impl.o
 obj-y += i2c.o

arch/powerpc/cpu/mpc5xxx/cache.c

+/*
+ * This file contains stub implementation of
+ *   invalidate_dcache_range()
+ *   flush_dcache_range()
+ *
+ * SPDX-License-Identifier:	GPL-2.0+
+ */
+
+void invalidate_dcache_range(unsigned long start, unsigned long stop)
+{
+}
+
+void flush_dcache_range(unsigned long start, unsigned long stop)
+{
+}