arm: Update our 'ret' assembler macro slightly

We only support cores that do Thumb-1 or later.  So we add a comment to
explain this and remove the architecture test.

Cc: Albert ARIBAUD <albert.u.boot@aribaud.net>
Cc: Mans Rullgard <mans@mansr.com>
Signed-off-by: Tom Rini <trini@konsulko.com>