mmc: meson-mx-sdhc: Fix building with CONFIG_MMC_MESON_MX_SDHC=m

mmc: meson-mx-sdhc: Fix building with CONFIG_MMC_MESON_MX_SDHC=m

For an x86_64 allmodconfig build Stephen reports that building
meson-mx-sdhc-clkc.o warns that MODULE_LICENSE is missing and when
linking meson_mx_sdhc_register_clkc cannot be found.

Compile the MMC controller driver together with the build-in clock
controller driver into one module rather than using two separate
modules to fix these issues.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Link: https://lore.kernel.org/r/20200518060811.1499962-1-martin.blumenstingl@googlemail.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>