Input: silead_gsl1680 - document firmware-name, fix implementation

The driver has supported touchscreen-fw-name to specify the firmware to
load since it has been merged, but this was omitted from the dt-binding
documentation.

During review of adding touchscreen-fw-name to the binding documentation
it was brought up that there is a standard property name called
"firmware-name" for this, which should be used.

Since there are no users of touchscreen-fw-name yet, this commit
adds documentation of "firmware-name" to the dt-binding documentation
and switches the driver over to use this.

This commit also makes the driver add a "silead/" prefix to the
firmware name from dt before calling request_firmware. That the
firmware files are stored under /lib/firmware/silead under Linux
is an implementation detail and does not belong in devicetree.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>