Remove the dbgu compatible strings in the UART submodule of the
flexcom for the proper SOC detection.

Fixes: 99c80833 (ARM: dts: at91: sam9x60: Add missing flexcom definitions)
Signed-off-by: Durai Manickam KR <durai.manickamkr@microchip.com>
Link: https://lore.kernel.org/r/20230712100042.317856-1-durai.manickamkr@microchip.comSigned-off-by: Arnd Bergmann <arnd@arndb.de>