StandaloneMmPkg: Add an AArch64 specific entry point library.

StandaloneMmPkg: Add an AArch64 specific entry point library.

The Standalone MM environment runs in S-EL0 in AArch64 on ARM Standard
Platforms and is initialised during the SEC phase. ARM Trusted firmware
in EL3 is responsible for initialising the architectural context for
S-EL0 and loading the Standalone MM image. The memory allocated to this
image is marked as RO+X. Heap memory is marked as RW+XN.

Certain actions have to be completed prior to executing the generic code
in the Standalone MM Core module. These are:

1. Memory permission attributes for each section of the Standalone MM
   Core module need to be changed prior to accessing any RW data.

2. A Hob list has to be created with information that allows the MM
   environment to initialise and dispatch drivers.

Furthermore, this module is responsible for handing over runtime MM
events to the Standalone MM CPU driver and returning control to ARM
Trusted Firmware upon event completion. Hence it needs to know the CPU
driver entry point.

This patch implements an entry point module that ARM Trusted Firmware
jumps to in S-EL0. It then performs the above actions before calling the
Standalone MM Foundation entry point and handling subsequent MM events.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Sughosh Ganu <sughosh.ganu@arm.com>
Signed-off-by: Achin Gupta <achin.gupta@arm.com>
Signed-off-by: Supreeth Venkatesh <supreeth.venkatesh@arm.com>
Reviewed-by: Achin Gupta <achin.gupta@arm.com>