configs: hihope_rzg2: Set correct MMC device for U-Boot env

configs: hihope_rzg2: Set correct MMC device for U-Boot env

Currently we set CONFIG_SYS_MMC_ENV_DEV=1 which is wrong:
  Loading Environment from MMC... MMC: no card present
  *** Warning - No block device, using default environment

This issue was introduced when we switched to using upstream Linux device
trees for the hihope boards which named the MMC devices differently.

Correct to dev 0 so that the U-Boot environment can be loaded from the
correct storage device.

Fixes: 18fb23b13158 ("ARM: dts: renesas: Switch to using upstream DT on 64bit Renesas SoCs")
Signed-off-by: Chris Paterson <chris.paterson2@renesas.com>
Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>