Make include/common.h usable by assembler code

Commit 70ebf316 factored out the ROUND() macro into include/common.h,
not realizing that the primary use of this macro on AT91 systems was
in start.S where common.h was not included, and could not be included
because it contains a lot of C code which the assembler doesn't
understand.

This patch wraps such code in common.h in a "#ifndef __ASSEMBLY__"
construct, and then adds an include to cpu/arm926ejs/start.S thus
solving the problem.

Signed-off-by: Wolfgang Denk <wd@denx.de>