[U-Boot] printf and R9 in ARM architecture

[Da Shi Cao]

In standalone applications, it seems that the execution of printf
function in U-Boot is dependent on the value of register R9, which is
setup in app_startup. But r9 can be used by your program at the
decision of gcc compilation which is not aware of this dependence.
This is especially the case if +O2 is used!

My version is U-Boot 2014.10

Regards,
Dashi Cao