mtd: physmap: Add Baikal-T1 physically mapped ROM support

Baikal-T1 Boot Controller provides an access to a RO storages, which are
physically mapped into the SoC MMIO space. In particularly there are
Internal ROM embedded into the SoC with a pre-installed firmware,
externally attached SPI flash (also accessed in the read-only mode) and a
memory region, which mirrors one of them in accordance with the currently
enabled system boot mode (also called Boot ROM).

This commit adds the Internal ROM support to the physmap driver of the MTD
kernel subsystem. The driver will create the Internal ROM MTD as long as
it is defined in the system dts file. The physically mapped SPI flash
region will be used to implement the SPI-mem interface. The mirroring
memory region won't be accessible directly since it's redundant due to
both bootable regions being exposed anyway.

Note we had to create a dedicated code for the ROMs since read from the
corresponding memory regions must be done via the dword-aligned addresses.

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Cc: Alexey Malahov <Alexey.Malahov@baikalelectronics.ru>
Cc: Pavel Parkhomenko <Pavel.Parkhomenko@baikalelectronics.ru>
Cc: Lee Jones <lee.jones@linaro.org>
Cc: linux-mips@vger.kernel.org
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200920111445.21816-1-Sergey.Semin@baikalelectronics.ru