common: fix regression on block cache init

m68k needs block cache list initialized after relocation.
Other architectures must not be involved.

Fixing regression related to:

commit 1526bcce0f7285087621e16e6720636d01839da8
	("common: add blkcache init")

Signed-off-by: Angelo Durgehello <angelo.dureghello@timesys.com>
diff --git a/common/board_r.c b/common/board_r.c
index 4f56c19..0bbeaa7 100644
--- a/common/board_r.c
+++ b/common/board_r.c
@@ -865,7 +865,7 @@
 #if defined(CONFIG_PRAM)
 	initr_mem,
 #endif
-#ifdef CONFIG_BLOCK_CACHE
+#if defined(CONFIG_M68K) && defined(CONFIG_BLOCK_CACHE)
 	blkcache_init,
 #endif
 	run_main_loop,