powerpc/sysdev: drop simple gpio

There is a config item CONFIG_SIMPLE_GPIO which
provides simple memory mapped GPIOs specific to powerpc.

However, the only platform which selects this option is
mpc5200, and this platform doesn't use it.

There are three boards calling simple_gpiochip_init(), but
as they don't select CONFIG_SIMPLE_GPIO, this is just a nop.

Simple_gpio is just redundant with the generic MMIO GPIO
driver which can be found in driver/gpio/ and selected via
CONFIG_GPIO_GENERIC_PLATFORM, so drop simple_gpio driver.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/bf930402613b41b42d0441b784e0cc43fc18d1fb.1572529632.git.christophe.leroy@c-s.fr