ARM: at91/Kconfig: add comment to at91sam9x5 family entry

Add comment to make it clear that several SoC are supported by
this generic entry.
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>

arch/arm/mach-at91/Kconfig

@@ -107,6 +107,12 @@ config ARCH_AT91SAM9X5
 	select HAVE_FB_ATMEL
 	select HAVE_NET_MACB
 	select HAVE_AT91_DBGU0
+	help
+	  Select this if you are using one of Atmel's AT91SAM9x5 family SoC.
+	  This means that your SAM9 name finishes with a '5' (except if it is
+	  AT91SAM9G45!).
+	  This support covers AT91SAM9G15, AT91SAM9G25, AT91SAM9X25, AT91SAM9G35
+	  and AT91SAM9X35.
 
 config ARCH_AT91X40
 	bool "AT91x40"