spi: bcm2835: Allow arbitrary number of slaves

Since commit 571e31f ("spi: bcm2835: Cache CS register value for
->prepare_message()"), the number of slaves has been limited by a
compile-time constant.  This was necessitated by statically-sized
arrays in the driver private data which contain per-slave register
values.

As suggested by Mark, move those register values to a per-slave
controller_state which is allocated on ->setup and freed on ->cleanup.
The limitation on the number of slaves is thus lifted.

Signed-off-by: Lukas Wunner <[email protected]>
Cc: Joe Burmeister <[email protected]>
Cc: Phil Elwell <[email protected]>
Link: https://lore.kernel.org/r/a847c01f09400801e74e0630bf5a0197591554da.1622150204.git.lukas@wunner.de
Signed-off-by: Mark Brown <[email protected]>

Author: l1k