powerpc: dts: t1040rdb: fix compatible string for Rev A boards

[ Upstream commit ae44f1c9 ]

It looks like U-Boot fails to start the kernel properly when the
compatible string of the board isn't fsl,T1040RDB, so stop overriding it
from the rev-a.dts.

Fixes: 5ebb7474

 ("powerpc: dts: t1040rdb: fix ports names for Seville Ethernet switch")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>