mmc: stm32_sdmmc2: manage vqmmc

The SDMMC IOs can be in an IO domain, that has to be enabled.
This is done by enabling vqmmc in the driver.
This has no impact on configurations not using an IO domain, the check
can then be executed on all platforms managing regulator, and the vqmmc
regulator enabled on all platforms having it in their DT.

Signed-off-by: Yann Gautier <yann.gautier@foss.st.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>