docs: add detail to assembly language guideline

Assembly language is rarely required and when it is, there are a lot of
helpers available to reduce the amount needed. Update the guidelines to
give pointers to them.

Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
Change-Id: Ic484e4ba57242594c351a9185ecb625d6e5dc223
1 file changed