board/beaglebone: fix boot on BeagleBone Black

Commit 68b5b79b has set the getty port to the default console but left
"ttyO0" in bootargs, in the U-Boot environment. Use "ttyS0", instead.

Also set loadaddr to 0x82000000 and fdtaddr to 0x88000000, replacing the
values that were valid for the ancient U-Boot and Linux pre-installed on
old boards but cause boot hangs with the current versions.

Signed-off-by: Carlos Santos <unixmania@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>