mx28: fix SPL code to make USB booting work

This patch fixes booting i.MX28 CPUs via USB download.
In this mode the CPU's bootrom implements a USB HID device that
accepts a bootstream.

When downloading the bootstream via USB, first the SPL code is
received and executed. Then the u-boot image is received and
called.

The USB bootmode is interrupt driven.

This patch fixes two things:

1) The ARM's fast interrupt mode is disabled when the SPL code
has been run. So save and restore the CPSR register.

2) Save and restore c1 control register: the exception vector
location needs to be set back to bootrom space to make the USB
interrupts work again. The SPL code needs to change this
option for the ram size probing.

Signed-off-by: Matthias Fuchs <matthias.fuchs@esd.eu>
Acked-by: Marek Vasut <marek.vasut@gmail.com>

arch/arm/cpu/arm926ejs/mx28/start.S

@@ -167,10 +167,15 @@ _reset:
 	 */
 	push	{r0-r12,r14}
 
+	/* save control register c1 */
+	mrc	p15, 0, r0, c1, c0, 0
+	push	{r0}
+
 	/*
-	 * set the cpu to SVC32 mode
+	 * set the cpu to SVC32 mode and store old CPSR register content
 	 */
 	mrs	r0,cpsr
+	push	{r0}
 	bic	r0,r0,#0x1f
 	orr	r0,r0,#0xd3
 	msr	cpsr,r0
@@ -185,6 +190,20 @@ _reset:
 
 	bl	board_init_ll
 
+	/*
+	 * restore bootrom's cpu mode (especially FIQ)
+	 */
+	pop	{r0}
+	msr	cpsr,r0
+
+	/*
+	 * restore c1 register
+	 * (especially set exception vector location back to
+	 * bootrom space which is required by bootrom for USB boot)
+	 */
+	pop	{r0}
+	mcr	p15, 0, r0, c1, c0, 0
+
 	pop	{r0-r12,r14}
 	bx	lr