arm: dts: aspeed: Update SPI flash node settings

For both AST2500 and AST2600, there are three
SPI controllers, FMC(Firmware Memory Controller),
SPI1 and SPI2. The clock source is HCLK. Following
is the basic information for ASPEED SPI controller.

AST2500:
  - FMC:
      CS number: 3
      controller reg: 0x1e620000 - 0x1e62ffff
      decoded address: 0x20000000 - 0x2fffffff

  - SPI1:
      CS number: 2
      controller reg: 0x1e630000 - 0x1e630fff
      decoded address: 0x30000000 - 0x37ffffff

  - SPI2:
      CS number: 2
      controller reg: 0x1e631000 - 0x1e631fff
      decoded address: 0x38000000 - 0x3fffffff

AST2600:
  - FMC:
      CS number: 3
      controller reg: 0x1e620000 - 0x1e62ffff
      decoded address: 0x20000000 - 0x2fffffff

  - SPI1:
      CS number: 2
      controller reg: 0x1e630000 - 0x1e630fff
      decoded address: 0x30000000 - 0x3fffffff

  - SPI2:
      CS number: 3
      controller reg: 0x1e631000 - 0x1e631fff
      decoded address: 0x50000000 - 0x5fffffff

Signed-off-by: Chin-Ting Kuo <chin-ting_kuo@aspeedtech.com>