memory: emif: Add ifdef CONFIG_DEBUG_FS guard for emif_debugfs_[init|exit]

Add ifdef CONFIG_DEBUG_FS guard for emif_debugfs_[init|exit], and adds stub
functions for the case CONFIG_DEBUG_FS is not set.

When CONFIG_DEBUG_FS is enabled, debugfs_create_dir and debugfs_create_file
return NULL on failure, fix it.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by : Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>