ARM: dts: am335x-phycore-som: Remove superseded/invalid GPMC NAND type.

According to docu and dtschema check, 'gpmc,device-nand = true' is
no longer valid, so remove it.

Signed-off-by: Steffen Hemer <s.hemer@phytec.de>
Message-Id: <20230214132302.39087-8-s.hemer@phytec.de>
Signed-off-by: Tony Lindgren <tony@atomide.com>