MIPS: ath25: register AR5312 flash controller

AR5312 SoC flash controller maps the flash content to memory and
translates the memory access operations to the flash access operations.
Such controller is fully supported by the physmap-flash driver.

Signed-off-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>R5312 SoC flash
Cc: Linux MIPS <linux-mips@linux-mips.org>
Patchwork: https://patchwork.linux-mips.org/patch/8245/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>