clk: imx8mn: drop of_match_ptr from of_device_id table

The driver can match only via the DT table so the table should be always
used and the of_match_ptr does not have any sense (this also allows ACPI
matching via PRP0001, even though it might be not relevant here).  This
fixes compile warning (!CONFIG_OF && !CONFIG_MODULES):

    drivers/clk/imx/clk-imx8mn.c:592:34: warning:
        ‘imx8mn_clk_of_match’ defined but not used [-Wunused-const-variable=]

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>