arm: vexpress64: Enable SYSRESET and SYSRESET_PSCI

arm: vexpress64: Enable SYSRESET and SYSRESET_PSCI

Select SYSRESET on Vexpress64 to enable system reset to support other
features, such as capsule-on-disk. Select SYSRESET_PSCI if PSCI is
inferred from the firmware (via ARM_PSCI_FW).
Select ARM_SMCCC for Vexpress64 boards which in turn selects
ARM_PSCI_FW.

The sysreset uclass unconditionally implements a reset_cpu() function.
Remove the empty reset_cpu() in vexpress64 board code.

Signed-off-by: Debbie Horsfall <debbie.horsfall@arm.com>