arm: Make u-boot-arm.h a private header

Rather than including this arch-specific header file in common.h, include
it from within arm's u-boot.h header.

Also drop the comment about something to be fixed. It has been there
forever and it is not clear what it means.

Signed-off-by: Simon Glass <sjg@chromium.org>

arch/arm/include/asm/u-boot-arm.h

@@ -13,6 +13,8 @@
 #ifndef _U_BOOT_ARM_H_
 #define _U_BOOT_ARM_H_	1
 
+#ifndef __ASSEMBLY__
+
 /* for the following variables, see start.S */
 extern ulong IRQ_STACK_START;	/* top of IRQ stack */
 extern ulong FIQ_STACK_START;	/* top of FIQ stack */
@@ -59,4 +61,6 @@ void do_fiq(struct pt_regs *pt_regs);
 void do_irq(struct pt_regs *pt_regswq);
 #endif
 
+#endif /* __ASSEMBLY__ */
+
 #endif	/* _U_BOOT_ARM_H_ */