rockchip: load env from boot MMC device

Currently, if the environment is stored on an MMC device, the device
number is hardcoded by CONFIG_SYS_MMC_ENV_DEV. This is problematic
because many boards can choose between booting from an SD card or a
removable eMMC. For example, the Rock64 defconfig sets
CONFIG_SYS_MMC_ENV_DEV=1, which corresponds to the SD card. If an eMMC
is used as the boot device and no SD card is installed, it is impossible
to save the environment.

To avoid this problem, we can choose the environment MMC device based on
the boot device. The theobroma-systems boards already contain code to do
this, so this commit simply moves it to the common Rockchip board file,
with some refactoring. I also removed another implementation of
mmc_get_env_dev() from tinker_rk3288 that performed MMC boot device
detection by reading a bootrom register.

This has been tested on a Rock64v2.

Signed-off-by: Ben Wolsieffer <benwolsieffer@gmail.com>
Reviewed-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>