scripts/tags.sh: handle OMAP platforms properly

When SUBARCH is "omap1" or "omap2", plat-omap/ directory must be
indexed.  Handle this special case properly.

While at it, check if mach- directory exists at all.

Link: http://lkml.kernel.org/r/20161202122148.15001-1-joe.skb7@gmail.com
Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Cc: Michal Marek <mmarek@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>