hw: m25p80: allow write_enable latch get/set

The write_enable latch property is not currently exposed.
This commit makes it a modifiable property.

Signed-off-by: Iris Chen <irischenlj@fb.com>
Acked-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Francisco Iglesias <frasse.iglesias@gmail.com>
Message-Id: <20220513055022.951759-1-irischenlj@fb.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>