RISC-V/PlatformPkg: Build DeviceTree and use that in SEC

OpenSBI uses the device tree for platform specific initialization, so we
need to have it already in SEC.

Cc: Daniel Schaefer <daniel.schaefer@hpe.com>
Cc: Abner Chang <abner.chang@hpe.com>
Cc: Sunil V L <sunilvl@ventanamicro.com>
Reviewed-by: Abner Chang <abner.chang@hpe.com>

Signed-off-by: Daniel Schaefer <daniel.schaefer@hpe.com>