doc: Reorder coding guidelines document
doc: Reorder coding guidelines document

This patch attempts to make the guidelines clearer by reordering
the sections and grouping similar topics.

Change-Id: I1418d6fc060d6403fe3e1978f32fd54b8793ad5b
Signed-off-by: Paul Beesley <8414e4843aab463885be16eab9f09388385653d9@arm.com>

achingupta/arm-trusted-firmware,achingupta/arm-trusted-firmware