am335x: Switch to CONFIG_SKIP_LOWLEVEL_INIT from guarding SPL or NOR_BOOT

In the case of SPL or NOR_BOOT (no SPL involved) we need to include
certain code in the build.  Use !CONFIG_SKIP_LOWLEVEL_INIT rather than
CONFIG_SPL_BUILD || CONFIG_NOR_BOOT to make the code clearer, and to
make supporting XIP QSPI boot clearer in the code.

Signed-off-by: Tom Rini <trini@ti.com>
Reviewed-by: Wolfgang Denk <wd@denx.de>
diff --git a/board/silica/pengwyn/Makefile b/board/silica/pengwyn/Makefile
index c8b4f9a..804ac37 100644
--- a/board/silica/pengwyn/Makefile
+++ b/board/silica/pengwyn/Makefile
@@ -6,7 +6,7 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
-ifeq ($(CONFIG_SPL_BUILD)$(CONFIG_NOR_BOOT),y)
+ifeq ($(CONFIG_SKIP_LOWLEVEL_INIT),)
 obj-y	:= mux.o
 endif