xtensa: move vecbase SR initialization to _startup

Instead of initializing vecbase in initialize_mmu macro, which may be
expanded either in Image.elf reset vector hadler or in the kernel
head.S, both times only when CONFIG_MMU is enabled, do this
initialization once in _startup function.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

arch/xtensa/kernel/head.S

@@ -112,6 +112,11 @@ ENTRY(_startup)
 
 	movi	a0, 0
 
+#if XCHAL_HAVE_VECBASE
+	movi    a2, VECBASE_RESET_VADDR
+	wsr	a2, vecbase
+#endif
+
 	/* Clear debugging registers. */
 
 #if XCHAL_HAVE_DEBUG