x86: doc: Mention qemu-x86_64 support

Currently only 32-bit U-Boot for QEMU x86 is documented. Mention
the 64-bit support.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
1 file changed