ARM plat: add build option to unlock access to non-secure timer

ARM plat: add build option to unlock access to non-secure timer

Currently, Trusted Firmware on ARM platforms unlocks access to the
timer frame registers that will be used by the Non-Secure world. This
unlock operation should be done by the Non-Secure software itself,
instead of relying on secure firmware settings.

This patch adds a new ARM specific build option 'ARM_CONFIG_CNTACR'
to unlock access to the timer frame by setting the corresponding
bits in the CNTACR<N> register. The frame id <N> is defined by
'PLAT_ARM_NSTIMER_FRAME_ID'. Default value is true (unlock timer
access).

Documentation updated accordingly.

Fixes ARM-software/tf-issues#170

Change-Id: Id9d606efd781e43bc581868cd2e5f9c8905bdbf6