dts: add gd32 fmc flash memory info

There are three types GD32 FMC.

GD32 FMC v1: its flash memory has 1 bank, page size is equal in the
bank, flash size is smaller than 512KB.

GD32 FMC v2: its flash memory has 2 banks. Page size equal within the
same bank but different between banks. Flash size can be up to 3072KB.
FMC v2 has two registers to control bank0 and bank1 separately.

GD32 FMC v3: its flash memory has 2 banks, use sector size as the
minimum operating unit, the sector size is not equal.

Signed-off-by: HaiLong Yang <hailong.yang@brainco.cn>