net: ixp4xx_hss: Check features using syscon

net: ixp4xx_hss: Check features using syscon

If we access the syscon (expansion bus config registers) using the
syscon regmap instead of relying on direct accessor functions,
we do not need to call this static code in the machine
(arch/arm/mach-ixp4xx/common.c) which makes things less dependent
on custom machine-dependent code.

Look up the syscon regmap and handle the error: this will make
deferred probe work with relation to the syscon.

Select the syscon in Kconfig and depend on OF so we know that
all we need will be available.

Cc: David S. Miller <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: netdev@vger.kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Link: https://lore.kernel.org/r/20220211223238.648934-10-linus.walleij@linaro.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>