configs: am64x_evm: Move CONFIG_SYS_MMC_ENV_DEV and CONFIG_SYS_MMC_ENV_PART to defcon...

configs: am64x_evm: Move CONFIG_SYS_MMC_ENV_DEV and CONFIG_SYS_MMC_ENV_PART to defconfig files and enable configs to save env in eMMC and FAT write.

Kconfig symbols for SYS_MMC_ENV_DEV and SYS_MMC_ENV_PART have been added by
commit cd494e706c8c57232a3082f5e4d2f9fd3ad47bb9. Therefore, move the
definitions of configs to corresponding board defconfig files and enable
configs to save env in eMMC.

Also enable config for FAT write in U-Boot.

Fixes: 1813b74ddf37 ("board: ti: am64x: Add board support for am64x evm")
Signed-off-by: Aswath Govindraju <a-govindraju@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Link: https://lore.kernel.org/r/20210726152807.22991-5-a-govindraju@ti.com