global_data: Remove comment of reserved arch-specific GD flags

global_data: Remove comment of reserved arch-specific GD flags

With the removal of the x86 specific GD flags, there are no arch-
specific GD flags any more. Let's remove the comment about reserving the
upper 16 bits for arch-specific flags in the common header. This gives
us more flexibility with the usage of the GD flags.

As a matter of fact, we are already using more than 16 bits for common
GD flags (with the addition of GD_FLG_WDT_READY).

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Bin Meng <bmeng.cn@gmail.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@konsulko.com>
Cc: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>