greybus: arche-platform: Add support for SPI bus sharing for Mihi

greybus: arche-platform: Add support for SPI bus sharing for Mihi

In case of Mihi, SPI bus is shared between APB1 and APB2
SPI ROMs, so their FW flashing must be sequential and
arche-platform driver should make sure that they are mutual
exclusive in nature.

So this patch adds certain restrictions to the user of the
arche-platform driver,

 - User can no longer flash APB1 and APB2 SPI ROM in parallel
 - SPI bus becomes an resource, so user must claim it by moving
   respective APB device into FW_FLASHING mode and release it
   by exiting FW_FLASHING mode. User can exit FW_FLASHING mode by
   switching to any other modes (ACTIVE, OFF, STANDBY).
 - If APB1 is in FW_FLASHING mode, APB2 can no longer enter into
   FW_FLASHING mode. User will get -EBUSY.

Having said that, while APB1 is into FW_FLASHING mode,
APB2 can independently boot from its own SPI ROM.

Testing Done: Tested by simulating usecase on EVT2.
 - Made sure that APB1 and APB2 FW_FLASHING mode is mutual exclusive
   in nature. Confirmed that an attempt on second device return -EBUSY.
 - Added simulating code, where printed state of dummy gpio for
   spi-en and verified that it shows right pin status for both APBs

Signed-off-by: Vaibhav Hiremath <vaibhav.hiremath@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>