ARM's generic timer[1] picks up it's graycode from GTC. However, the
frequency of the GTC is supposed to be programmed in CNTFID0[2]
register.

In K3, architecture, GTC provides a central time to many parts of the
SoC including graycode to the generic timer in the ARMv8 subsystem.
However, due to the central nature and the need to enable the counter
early in the boot process, the R5 based bootloader enables GTC and
programs it's frequency based on central needs of the system. This
may not be a constant 200MHz based on the system. The bootloader is
supposed to program the FID0 register with the correct frequency it
has sourced for GTC from the central system controller, and TF-A is
supposed to use that as the frequency for it's local timer.

A mismatch in programmed frequency and what we program for generic
timer will, as we can imagine, all kind of weird mayhem.

So, check the CNTFID0 register, if it is 0, warn and use the default
frequency to continue the boot process.

While at it, we can also check CNTCR register to provide some basic
diagnostics to make sure that we don't have OS folks scratch their
heads. Even though this is used during cpu online operations, the cost
of this additional check is minimal enough for us not to use #ifdeffery
with DEBUG flags.

[1] https://developer.arm.com/documentation/100095/0002/generic-timer/generic-timer-register-summary/aarch64-generic-timer-register-summary
[2] https://developer.arm.com/docs/ddi0595/h/external-system-registers/cntfid0
[3] https://developer.arm.com/docs/ddi0595/h/external-system-registers/cntcr

Signed-off-by: Nishanth Menon <nm@ti.com>
Change-Id: Ib03e06788580f3540dcb1a11677d0d6d398b2c9f