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.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/fsl_lpuart.c

@@ -240,7 +240,7 @@
 
 #define DRIVER_NAME	"fsl-lpuart"
 #define DEV_NAME	"ttyLP"
-#define UART_NR		6
+#define UART_NR		8
 
 /* IMX lpuart has four extra unused regs located at the beginning */
 #define IMX_REG_OFF	0x10