[MTD] [NOR] Add "Spansion" to MTD_CFI_AMDSTD kconfig menu description

This long overdue trivial change to the MTD_CFI_AMDSTD kconfig menu
description is intended to help clarify that this option also supports
Spansion flash devices.

Signed-off-by: George G. Davis <gdavis@mvista.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>