firmware-utils: ptgen: add SiFive GPT partition support

firmware-utils: ptgen: add SiFive GPT partition support

The SiFive RISC-V SoCs use two special partition types in the boot process.
As a first step, the ZSBL (zero-stage bootloader) in the CPU looks for a
partition with a GUID of 5B193300-FC78-40CD-8002-E86C45580B47 to load the
first-stage bootloader - which in OpenWrt's case is an SPL image. The FSBL
(SPL) then looks for a partition with a GUID of
2E54B353-1271-4842-806F-E436D6AF6985 to load the SSBL which is usually an
u-boot.

With ptgen already supporting GPT partition creation, add the required GUID
types and name them accordingly to be invoked with the '-T <GPT partition
type>' parameter.

Signed-off-by: Zoltan HERPAI <wigyori@uid0.hu>