clk: mmp2: add SP clock

The "security processor", sometimes referred to as "wireless trusted
module" or "generic encrypt unit" is a low-power core present on MMP2,
that has nothing to do with security, wireless, trust or encryption.

On an OLPC machine it runs CForth and serves as a keyboard controller:
http://dev.laptop.org/git/users/wmb/cforth/tree/src/app/arm-xo-1.75/ps2.fth



The register address was obtained from the OLPC kernel, since the
datasheet seems to be the Marvell's most important business secret.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Acked-by: Stephen Boyd <sboyd@kernel.org>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>