Platform: Remove 'TFM_UNPRIV_DATA' section

This section is not used. Remove this section and related platform
sources, and remove some unprivileged code objects out of the
unprivileged code section because they are not used by unprivileged
logic.

Change-Id: I9915cdd2d90a80cb52c357e0d759de2854e971de
Signed-off-by: Ken Liu <Ken.Liu@arm.com>