ARM: dts: imx6ul: add wdog3

The i.MX6UL has a WDOG3 located at start address 0x021E0000 in the
AIPS-2 memory region [1].

[1] i.MX 6UltraLite Applications Processor Reference Manual, Rev. 1,
    04/2016, Table-2-3 AIPS-2 memory map, p. 166

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>