clk: imx8: Add DSP related clocks

i.MX8QXP contains Hifi4 DSP. There are four clocks
associated with DSP:
  * dsp_lpcg_core_clk
  * dsp_lpcg_ipg_clk
  * dsp_lpcg_adb_aclk
  * ocram_lpcg_ipg_clk

Signed-off-by: Daniel Baluta <daniel.baluta@nxp.com>
Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>