arm: Add declarations to avoid needing to include headers

At present common.h includes various ARM-specific headers. In preparation
for dropping this, add a few explicit declarations.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/arm/include/asm/u-boot-arm.h b/arch/arm/include/asm/u-boot-arm.h
index 3a0d0b8..ef4fca6 100644
--- a/arch/arm/include/asm/u-boot-arm.h
+++ b/arch/arm/include/asm/u-boot-arm.h
@@ -47,6 +47,8 @@
 void	udelay_masked		(unsigned long usec);
 
 /* calls to c from vectors.S */
+struct pt_regs;
+
 void bad_mode(void);
 void do_undefined_instruction(struct pt_regs *pt_regs);
 void do_software_interrupt(struct pt_regs *pt_regs);