doc: fix README.standalone

Since commit fe1378a961e5 (ARM: use r9 for gd), r9 register
is used on ARM to hold global_data.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Jeroen Hofstee <jeroen@myspectrum.nl>
1 file changed