arm: Refactor bootm to reduce #ifdefs

With fewer #ifdefs the code is more readable and more of the code is
compiled for all boards. Add defines in the header file to control
what features are enabled, and then use if() instead of #ifdef.

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

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

@@ -54,8 +54,6 @@ int	arch_early_init_r(void);
 int	board_init(void);
 int	dram_init (void);
 void	dram_init_banksize (void);
-void	setup_serial_tag (struct tag **params);
-void	setup_revision_tag (struct tag **params);
 
 /* cpu/.../interrupt.c */
 int	arch_interrupt_init	(void);