dm: timer: normalise SPL and TPL support

To fully support DM timer in SPL and TPL, we need a few things cleaned
up and normalised:
- inclusion of the uclass and drivers should be an all-or-nothing
  decision for each stage and under control of $(SPL_TPL_)TIMER
  instead of having the two-level configuration with TIMER and
  $(SPL_TPL_)TIMER_SUPPORT
- when $(SPL_TPL_)TIMER is enabled, the ARMv8 generic timer code can
  not be compiled in

This normalises configuration to $(SPL_TPL_)TIMER and moves the config
options to drivers/timer/Kconfig (and cleans up the collateral damage
to some defconfigs that had SPL_TIMER_SUPPORT enabled).

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>