clk: Add TI-Nspire clock drivers

This patch adds a basic clock driver for the TI-Nspire calculator
series.

Changes from v1:
* Removed filename in header comment
* Removed unnecessary #undef EXTRACT statement

Signed-off-by: Daniel Tang <dt.tangr@gmail.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>
[mturquette@linaro.org: fixed $SUBJECT and changelog max width]