powerpc, 8xx: Move cache function into C files

Avoid unnecessary assembly functions when they can easily be written
in C.

Also remove dc_read() as it is nowhere referenced

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
diff --git a/arch/powerpc/cpu/mpc8xx/Makefile b/arch/powerpc/cpu/mpc8xx/Makefile
index b40bffb..40f3892 100644
--- a/arch/powerpc/cpu/mpc8xx/Makefile
+++ b/arch/powerpc/cpu/mpc8xx/Makefile
@@ -14,3 +14,4 @@
 obj-y	+= interrupts.o
 obj-$(CONFIG_CMD_REGINFO) += reginfo.o
 obj-y	+= speed.o
+obj-y	+= cache.o