fs: jffs2: add initial support for reading jffs2 ...

Import the jffs2 filesystem code from Linux v5.5-rc1 and convert it
to work with barebox.

Writing is *not* supported.

Testing was done with a n25q256a-compatible QuadSPI chip on a
SoCFPGA-based Socrates board.

Testing was done with a combination of:
	user@somelinuxhost: mkfs.jffs2 --eraseblock=4 -d fs/jffs2/ -o jffs2.img
	root@target:~ flash_erase -j /dev/mtd5 0 0
	root@target:~ dd if=jffs2.img of=/dev/mtd5
	barebox@EBV SOCrates:/ mount /dev/mtd0.data
	(...)
	mounted /dev/mtd0.data on /mnt/mtd0.data

Signed-off-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>