drivers/intel/fsp2_0: Allow `mp_startup_all_cpus()` to run serially

drivers/intel/fsp2_0: Allow `mp_startup_all_cpus()` to run serially

As per MP service specification, EDK2 is allowed to specify the mode
in which a 'func' routine should be executed on APs.

`SingleThread` sets to 'true' meaning to execute the function one by
one (serially) or sets to 'false' meaning to execute the function
simultaneously.

MP service API `StartupAllAPs` was designed to pass such options as
part of function argument.

But another MP service API `StartupAllCPUs` doesn't specify any such
requirement. Running the `func` simultaneously on APs results in
a coherency issue (hang while executing `func`) due to lack of
acquiring a spin lock while accessing common data structure in
multiprocessor environment.

BUG=b:199246420

Change-Id: Ia95d11408f663212fd40daa9fd9b0881a07f1ce7
Signed-off-by: Subrata Banik <subrata.banik@intel.com>
Signed-off-by: Ronak Kanabar <ronak.kanabar@intel.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/57343
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Maulik V Vaghela <maulik.v.vaghela@intel.com>
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>