ARM: tegra: add tegra_timer clock

This undoes commit 20f46658 "ARM: tegra: remove tegra_timer from
tegra_list_clks" by bringing back the tegra_timer clock. tegra_timer is
indeed a clock (hidden by the PERIPH_CLK macro) which should be added
to the tegra_list_clks.

The above commit caused tegra_init_timer() failing to get the clk
reference.

Signed-off-by: Sivaram Nair <sivaramn@nvidia.com>
[swarren: added the reverted commit's subject to this patch description]
Signed-off-by: Stephen Warren <swarren@nvidia.com>