arm: am335x: add Kconfig range attribute to prevent invalid CONS_INDEX

The help message in board/ti/am335x/Kconfig says AM335x has
6 UARTs, so the valid number for CONFIG_CONS_INDEX is from 1 to 6.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Tom Rini <trini@ti.com>

board/ti/am335x/Kconfig

@@ -22,6 +22,7 @@ config SYS_CONFIG_NAME
 
 config CONS_INDEX
 	int "UART used for console"
+	range 1 6
 	default 1
 	help
 	  The AM335x SoC has a total of 6 UARTs (UART0 to UART5 as referenced