ti/beaglebone: Define arch for omap-header build

Required to add rules.h as default include, otherwise we get error:

   ./src/include/rules.h:128:5: error:
      "__COREBOOT_ARM_ARCH__" is not defined [-Werror=undef]

Previously, rules.h was not included in omap-header build at all.

Change-Id: I75265916856f2f21f7966619ea65d63acd599e2f
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/17746
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/cpu/ti/am335x/Makefile.inc

@@ -21,6 +21,8 @@ endif
 $(call add-class,omap-header)
 $(eval $(call create_class_compiler,omap-header,arm))
 
+omap-header-generic-ccopts += -D__COREBOOT_ARM_ARCH__=7
+
 real-target: $(obj)/MLO
 
 header_ld := $(call src-to-obj,omap-header,$(dir)/header.ld)