efi_loader: Add check for event log passed from firmware

Platforms may have support to measure their initial firmware components
and pass the event log to u-boot. The event log address can be passed
in property tpm_event_log_addr and tpm_event_log_size of the tpm node.
Platforms may choose their own specific mechanism to do so. A weak
function is added to check if even log has been passed to u-boot
from earlier firmware components. If available, the eventlog is parsed
to check for its correctness and further event logs are appended to the
passed log.

Signed-off-by: Ruchika Gupta <ruchika.gupta@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Tested-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>