xtensa: nommu: fix default memory start address

RAM starts at 0x60000000 on noMMU cores, not at 0x40000000. Fix the
default.
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

arch/xtensa/Kconfig

@@ -408,7 +408,7 @@ config DEFAULT_MEM_START
 	hex "Physical address of the default memory area start"
 	depends on PLATFORM_WANT_DEFAULT_MEM
 	default 0x00000000 if MMU
-	default 0x40000000 if !MMU
+	default 0x60000000 if !MMU
 	help
 	  This is a fallback start address of the default memory area, it is
 	  used when no physical memory size is passed through DTB or through