m68knommu: improve short help of m68knommu/Kconfig/RAMSIZE for '0' case

While it is explained in the long help text, meaning of '0' for RAMSIZE
is easily overlooked because is not mentioned in the short help text.
Add that.
Signed-off-by: NPhilippe De Muyter <phdm@macqel.be>
Signed-off-by: NGreg Ungerer <gerg@uclinux.org>

arch/m68knommu/Kconfig

@@ -566,7 +566,7 @@ config RAMBASE
 	  processor address space.
 
 config RAMSIZE
-	hex "Size of RAM (in bytes)"
+	hex "Size of RAM (in bytes), or 0 for automatic"
 	default "0x400000"
 	help
 	  Define the size of the system RAM. If you select 0 then the