README.arm-relocation: get relocated address in gdb

When your emulator is connected at reset (or is used to load u-boot)
it is possible to get the relocation address from the gd->relocaddr
since gd is always in r8 (on ARM) it is addressable before the
gdb has remapped symbols.

Document this alternate method in-line with the original method
written by Heiko Schocher.

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