mmc: meson-gx: 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 is not relevant here).  This fixes
compile warning (!CONFIG_OF && !CONFIG_MODULES):

    drivers/mmc/host/meson-gx-mmc.c:1252:34: warning:
        ‘meson_mmc_of_match’ defined but not used [-Wunused-const-variable=]

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Kevin Hilman <khilman@baylibre.com>
Link: https://lore.kernel.org/r/20201103163046.14336-1-krzk@kernel.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>