tty: serial: fsl_lpuart: increase maximum uart_nr to eight

tty: serial: fsl_lpuart: increase maximum uart_nr to eight

Some SoCs like the i.MX93 have aliases for up to eight UARTs, see:
https://github.com/torvalds/linux/blob/v6.1/arch/arm64/boot/dts/freescale/imx93.dtsi#L31-L38

Increase UART_NR from 6 to 8 to support lpuart7 and lpuart8 and avoid
initialization failures like the following:

[    0.837146] fsl-lpuart 42690000.serial: serial6 out of range
[    0.842814] fsl-lpuart: probe of 42690000.serial failed with error -22

Signed-off-by: Nate Drude <nate.d@variscite.com>
Reviewed-by: Sherry Sun <sherry.sun@nxp.com>
Link: https://lore.kernel.org/r/20221222145634.2217793-1-nate.d@variscite.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>