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 7d080773347c1f6e0e896d9284134a2a411155d6. 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: 33b7258947f4 ("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