i.MX: Add the CLK_IPG_HIGH clock

i.MX: Add the CLK_IPG_HIGH clock

EPIT, GPT and other i.MX timers are using "abstract" clocks among which
a CLK_IPG_HIGH clock.

On i.MX25 and i.MX31 CLK_IPG and CLK_IPG_HIGH are mapped to the same clock
but on other SOC like i.MX6 they are mapped to distinct clocks.

This patch add the CLK_IPG_HIGH to prepare for SOC where these 2 clocks are
different.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Jean-Christophe Dubois <jcd@tribudubois.net>
Message-id: 224bf650194760284cb40630e985867e1373276a.1456868959.git.jcd@tribudubois.net
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>