ARM: dts: aspeed-g4: Correct VUART IRQ number

This should have always been 8.

Fixes: db4d6d9d

 ("ARM: dts: aspeed: Correctly order UART nodes")
Cc: stable@vger.kernel.org
Signed-off-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>