boards: riscv: Add SiPeed Longan Nano platform

SiPeed Longan Nano is a minimal development board
based on GigaDevice's RISC-V processor.
There are 2 board variations.

longan_nano:      GDGD32VF103CBT6 (128K Flash/32K SRAM)
longan_nano_lite: GDGD32VF103C8T6 ( 64K Flash/20K SRAM)

Signed-off-by: TOKITA Hiroshi <tokita.hiroshi@gmail.com>
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