Adding cross compile for armv7 to docs

[Peter Polgar]

Hi,

I have recently built U-Boot for an armv7 board (OrangePi PC). It took me
some googling to find the dependencies needed to cross compile on Ubuntu.
I am thinking about adding the info to the documentation here:
https://github.com/u-boot/u-boot/blob/master/doc/build/gcc.rst#debian-based

I have a few questions though:
Can I send a PR now or should I wait for the next merge window?
Is it ok to commit to github.com or should it go to source.denx.de instead?

Thanks,
Peter