Nuvoton WPCM450 FIU SPI flash controller

Merge series from Jonathan Neuschäfer <j.neuschaefer@gmx.net>:

This patchset adds DT bindings and a driver for the Flash Interface Unit
(FIU), the SPI flash controller in the Nuvoton WPCM450 BMC SoC. It
supports four chip selects, and direct (memory-mapped) access to 16 MiB
per chip. Larger flash chips can be accessed by software-defined SPI
transfers.

The existing NPCM7xx FIU driver is sufficitently incompatible with the
WPCM450 FIU that I decided to write a new driver.