gpio: fix up SPI submenu

- Relax dependencies on SPI_MASTER for drivers in the SPI menu
  that already has this dependency.
- Move out the expander that would be hidden for I2C access if
  SPI_MASTER was not selected. Tentatively create a separate
  menu for this.
- Move the ZX SoC driver to memory-mapped drivers, this must be
  a mistake and only worked because the system has an SPI master
  enabled at the same time.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>