arm: Allow skipping of low-level init with I-cache on

At present CONFIG_SKIP_LOWLEVEL_INIT prevents U-Boot from calling
lowlevel_init(). This means that the instruction cache is not enabled and
the board runs very slowly.

What is really needed in many cases is to skip the call to lowlevel_init()
but still perform CP15 init. Add an option to handle this.

Reviewed-by: Heiko Schocher <hs@denx.de>
Tested-on: smartweb, corvus, taurus, axm
Tested-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Andreas Bießmann <andreas@biessmann.org>
diff --git a/arch/arm/cpu/arm926ejs/start.S b/arch/arm/cpu/arm926ejs/start.S
index f05113d..959d1ed 100644
--- a/arch/arm/cpu/arm926ejs/start.S
+++ b/arch/arm/cpu/arm926ejs/start.S
@@ -101,11 +101,13 @@
 #endif
 	mcr	p15, 0, r0, c1, c0, 0
 
+#ifndef CONFIG_SKIP_LOWLEVEL_INIT_ONLY
 	/*
 	 * Go setup Memory and board specific bits prior to relocation.
 	 */
 	mov	ip, lr		/* perserve link reg across call */
 	bl	lowlevel_init	/* go setup pll,mux,memory */
 	mov	lr, ip		/* restore link */
+#endif
 	mov	pc, lr		/* back to my caller */
 #endif /* CONFIG_SKIP_LOWLEVEL_INIT */