common: add blkcache init

On m68k, block_cache list is relocated, but next and prev list
pointers are not adjusted to the relocated struct list_head address,
so the first iteration over the block_cache list hangs.

This patch initializes the block_cache list after relocation.

Signed-off-by: Angelo Durgehello <angelo.dureghello@timesys.com>
Reviewed-by: Eric Nelson <eric@nelint.com>

include/blk.h

@@ -113,6 +113,12 @@ struct blk_desc {
 	(PAD_SIZE(size, blk_desc->blksz))
 
 #if CONFIG_IS_ENABLED(BLOCK_CACHE)
+
+/**
+ * blkcache_init() - initialize the block cache list pointers
+ */
+int blkcache_init(void);
+
 /**
  * blkcache_read() - attempt to read a set of blocks from cache
  *