sifiveu: add new target for SiFive U-based boards

sifiveu: add new target for SiFive U-based boards

RISC-V is a new CPU architecture aimed to be fully free and open. This
target will add support for it, based on 5.15.

Supports running on:
 - HiFive Unleashed - FU540, first generation
 - HiFive Unmatched - FU740, current latest generation, PCIe

SD-card images are generated, where the partitions are required to have
specific type codes. As it is commonplace nowadays, OpenSBI is used as the
first stage, with U-boot following as the proper bootloader.

Specifications:

HiFive Unleashed:
 - CPU: SiFive FU540 quad-core RISC-V (U54, RV64IMAFDC or RV64GC)
 - Memory: 8Gb
 - Ethernet: 1x 10/100/1000
 - Console: via microUSB

HiFive Unmatched:
 - CPU: SiFive FU740 quad-core RISC-V (U74, RV64IMAFDCB or RV64GCB)
 - Memory: 16Gb
 - Ethernet: 1x 10/100/1000
 - USB: 4x USB 3.2
 - PCIe:  - 1x PCIe Gen3 x8
          - 1x M.2 key M (PCIe x4)
          - 1x M.2 Key E (PCIe x1 / USB2.0)
 - Console: via microUSB

Installation:
Standard SD-card installation via dd-ing the generated image to
an SD-card of at least 256Mb.

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