Platforms/RaspberryPi: Add Asset Tag support

Add support for configuring the Chassis Asset Tag in the UI as well as
UEFI Shell, and carry the configured value as the Asset Tag string
in SMBIOS Types 2 and 3.

To configure using the UEFI Shell, use 'setvar' command to read/write
the UEFI variable with GUID = gConfigDxeFormSetGuid and Name="AssetTag".
For example:

Shell> setvar AssetTag -guid CD7CC258-31DB-22E6-9F22-63B0B8EED6B5 -bs
-rt -nv =L"ABC123" =0x0000

This resolves this Github issue: https://github.com/pftf/RPi4/issues/54



Signed-off-by: Samer El-Haj-Mahmoud <samer.el-haj-mahmoud@arm.com>
Reviewed-by: Andrei Warkentin <andrey.warkentin@gmail.com>