board: stm32mp1: use CONFIG_SYS_MMC_ENV_DEV when available

Check whether user has explicitly defined the mmc device to use
in mmc_get_env_dev() with CONFIG_SYS_MMC_ENV_DEV.

On STMicroelectronics boards the used mmc device for environment is
the instance of boot device provided by the ROM code; the mmc instance
is configured by alias in device tree. The used partition is defined in
device tree with u-boot,mmc-env-partition = "ssbl".

This patch allows to override this selection for the support of customer
boards without alias; for example when SDMMC1 is not used and ENV in
mmc0=SDMMC2, user can force the value: CONFIG_SYS_MMC_ENV_DEV = 0.

On STMicroelectronics boards, the current behavior is kept with
CONFIG_SYS_MMC_ENV_DEV = -1.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>