misc: Replace `<assert.h>` includes with `<fwk_assert.h>`

This commit moves all inclusions of <assert.h> into <fwk_assert.h>,
which now includes it instead. This allows us to keep all assert-related
logic under one roof.

Change-Id: I4ec0d950dfd9264733645bc14da952b8c1619501
Signed-off-by: Chris Kay <chris.kay@arm.com>