gpio: mb86s7x: share with other SoCs as module

In order to reuse this driver for the Socionext Synquacer SC2A11 SoC,
which inherited this IP from Fujitsu, remove the ARCH_MB86S7X Kconfig
dependency, and revert the changes that prevent it from being built as
a module.

This reverts commits d65aa4b6 and
d5610e51

.

Cc: Geliang Tang <geliangtang@gmail.com>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
[Folded in module_platform_driver() fixup]
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>