[U-Boot] Please update doc/git-mailrc

Hello Andy Freming,

All the emails sent to your email address afleming@freescale.com have been bouncing.

If Andy Fleming afleming@freescale.com is no longer valid address, could you update the following line in doc/git-mailrc, please?

alias afleming Andy Fleming afleming@freescale.com

Best Regards Masahiro Yamada