fs: Move ls and read methods into ext4, fat

It doesn't make a lot of sense to have these methods in fs.c. They are
filesystem-specific, not generic code. Add each to the relevant
filesystem and remove the associated #ifdefs in fs.c.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@ti.com>
diff --git a/include/fat.h b/include/fat.h
index b28c3fd..9701323 100644
--- a/include/fat.h
+++ b/include/fat.h
@@ -213,4 +213,6 @@
 int fat_register_device(block_dev_desc_t *dev_desc, int part_no);
 
 int file_fat_write(const char *filename, void *buffer, unsigned long maxsize);
+int fat_read_file(const char *filename, void *buf, int offset, int len);
+void fat_close(void);
 #endif /* _FAT_H_ */