configs: j7200_evm_a72_defconfig: Re-enable MUX_MMIO at U-Boot proper

configs: j7200_evm_a72_defconfig: Re-enable MUX_MMIO at U-Boot proper

Commit under Fixes introduced support for Hyperflash boot but also disabled
MUX_MMIO support at U-Boot proper. MUX_MMIO is required at U-Boot proper
to setup the SERDES Lane Mapping as described by the device-tree. On J7200
SoC, PCIe functionality is broken since commit under fixes.

Fix this regression by re-enabling MUX_MMIO.

Fixes: 038f6faea9f0 ("configs: j7200_evm_*_defconfig: Add configs for Hyperflash boot")
Signed-off-by: Siddharth Vadapalli <s-vadapalli@ti.com>
Reviewed-by: Anurag Dutta <a-dutta@ti.com>