EmbeddedPkg/VirtualRealTimeClockLib: Fix SetTime issues

EmbeddedPkg/VirtualRealTimeClockLib: Fix SetTime issues

This patch fixes two issues below:
1. SCT SetTime_Func failures.
   - https://github.com/pftf/RPi4/issues/164
2. Using shell time and date commands to set time can't work.

The problem is that gRT->SetTime always returns EFI_INVALID_PARAMETER
error status.

The root cause is that LibSetTime() sets RtcEpochSeconds variable with
inconsistent attributes. One is without EFI_VARIABLE_NON_VOLATILE,
the other one is with EFI_VARIABLE_NON_VOLATILE. That caused that the
variable driver returns EFI_INVALID_PARAMETER. Per UEFI spec, if a
preexisting variable is rewritten with different attributes,
SetVariable() shall not modify the variable and shall return
EFI_INVALID_PARAMETER.

Therefore, the solution is to add EFI_VARIABLE_NON_VOLATILE attribute
to the first EfiSetVariable() call to make two calls consistent.

By the way, this patch also fix a minor issue with a debug message.

Signed-off-by: Sunny Wang <sunny.wang@arm.com>
Reviewed-by: Pete Batard <pete@akeo.ie>
Tested-by: Pete Batard <pete@akeo.ie>