mtd: spi-nor-core: Add a ->setup() hook

nor->setup() can be used by flashes to configure settings in case they
have any peculiarities that can't be easily expressed by the generic
spi-nor framework. This includes things like different opcodes, dummy
cycles, page size, uniform/non-uniform sector sizes, etc.

Move related declarations to avoid forward declarations.

Inspired by the Linux kernel's setup() hook.

Signed-off-by: Pratyush Yadav <p.yadav@ti.com>
Acked-by: Jagan Teki <jagan@amarulasolutions.com>