README.arm-relocation: get relocated address from bdinfo

The bdinfo command prints the relocaddr on ARM as it does
on PPC.

Update the debugging instructions for arm relocation to
reflect this fact rather than requiring that the user
rebuild the u-boot image using -DDEBUG.

Signed-off-by: Ben Gardiner <bengardiner@nanometrics.ca>
CC: Wolfgang Denk <wd@denx.de>
Acked-by: Heiko Schocher <hs@denx.de>