nds32: fix unused pmu_init warning

Fixes the build-time warning:
	board.c: At top level:
	board.c:106: warning: 'pmu_init' defined but not used

This makes the ifdef logic at the call site match the logic at the
function definition.
Signed-off-by: NMike Frysinger <vapier@gentoo.org>

arch/nds32/lib/board.c

@@ -103,6 +103,7 @@ static int nds32_pci_init(void)
 #endif /* CONFIG_CMD_PCI || CONFIG_PCI */
 
 #if defined(CONFIG_PMU) || defined(CONFIG_PCU)
+#ifndef CONFIG_SKIP_LOWLEVEL_INIT
 static int pmu_init(void)
 {
 #if defined(CONFIG_FTPMU010_POWER)
@@ -115,6 +116,7 @@ static int pmu_init(void)
 	return 0;
 }
 #endif
+#endif
 
 /*
  * Breathe some life into the board...