odroid: Update README with correct firmware link and XU4 support

The firmware from link [1] only works with U-Boot image that is no
bigger than 328KiB.  Using it with the default mainline U-Boot today
which is already around 500KiB is just not working.  Correct the link
to be hardkernel_1mb_uboot one [2], so that users can get mainline
U-Boot work out of box.

While at it, the README is updated to include XU4 support, like DTB file
name.

[1] https://github.com/hardkernel/u-boot/tree/odroidxu3-v2012.07/sd_fuse/hardkernel
[2] https://github.com/hardkernel/u-boot/tree/odroidxu3-v2012.07/sd_fuse/hardkernel_1mb_uboot



Signed-off-by: Shawn Guo <shawnguo@kernel.org>
Acked-by: Przemyslaw Marczak <p.marczak@samsung.com>
Reviewed-by: Anand Moon <linux.amoon@gmail.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>