imx: gpt: Add High frequency clock source support for GPT

Introduce a new configuration "CONFIG_MXC_GPT_HCLK". When it is set,
the GPT will select a high frequency clock as clock source.
Otherwise, the GPT will stay to use 32Khz OSC as clock source.

In the implementation, since only the GPT on i.MX6 series provide the
clock source option for 24Mhz OSC. For others (only i.MX5 and i.MX6
compile the driver), if the configuration is set, the perclk will be
selected as clock source.
MX6Q/D Rev 1.0 and MX6SL are special in the implementation, because they
don't have the 24Mhz OSC clock source option, so also select the perclk
for them. For MX6SL, we will set the OSC 24Mhz to perclk in CCM, so
eventually the clock comes from OSC 24Mhz.

Signed-off-by: Ye.Li <B37916@freescale.com>