Realtek SPI-NAND controller

Merge series from Chris Packham <chris.packham@alliedtelesis.co.nz>:

This series adds support for the SPI-NAND flash controller on the RTL9300
family of SoCs.

There are 2 physical chip selects which are called SPI_MST_CS0 and SPI_MST_CS1
in the datasheet. Via some pin-strapping these can be assigned to either the
SPI-NOR controller or the SPI-NAND controller. Which means you can end up with
the following permutations

  SPI-Flash
  Boot Model SPI_MST_CS0 SPI_MST_CS1
  ---------- ----------- -----------
  NOR x1     NOR-CS0     X
  NOR x2     NOR-CS0     NOR-CS1
  NAND x1    NAND-CS0    X
  NAND x2    NAND-CS0    NAND-CS1
  NOR+NAND   NOR-CS0     NAND-CS0