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/arm920t/start.S b/arch/arm/cpu/arm920t/start.S
index 69cabeb..3ada6d0 100644
--- a/arch/arm/cpu/arm920t/start.S
+++ b/arch/arm/cpu/arm920t/start.S
@@ -135,6 +135,7 @@
 	orr	r0, r0, #0x00001000	@ set bit 12 (I) I-Cache
 	mcr	p15, 0, r0, c1, c0, 0
 
+#ifndef CONFIG_SKIP_LOWLEVEL_INIT_ONLY
 	/*
 	 * before relocating, we have to setup RAM timing
 	 * because memory timing is board-dependend, you will
@@ -143,7 +144,7 @@
 	mov	ip, lr
 
 	bl	lowlevel_init
-
 	mov	lr, ip
+#endif
 	mov	pc, lr
 #endif /* CONFIG_SKIP_LOWLEVEL_INIT */