configs: am62x_evm_a53_defconfig: Disable semi-functional PSCI reset support

At this point, system shutdown is not supported by the DM firmware
that TF-A calls. As we can't de-select only this feature[1], declare
complete PSCI reset support as non-functional so that we don't signal
incomplete support to the OS via EFI runtime services. This makes
power-off under Linux work again when booting via EFI.

[1] https://uefi.org/specs/UEFI/2.9_A/08_Services_Runtime_Services.html?highlight=efiresetshutdown#resetsystem


Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>