drivers: soc: xilinx: fix firmware driver Kconfig dependency

commit d0384eed upstream.

The firmware driver is optional, but the power driver depends on it,
which needs to be reflected in Kconfig to avoid link errors:

aarch64-linux-ld: drivers/soc/xilinx/zynqmp_power.o: in function `zynqmp_pm_isr':
zynqmp_power.c:(.text+0x284): undefined reference to `zynqmp_pm_invoke_fn'

The firmware driver can probably be allowed for compile-testing as
well, so it's best to drop the dependency on the ZYNQ platform
here and allow building as long as the firmware code is built-in.

Fixes: ab272643

 ("drivers: soc: xilinx: Add ZynqMP PM driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20200408155224.2070880-1-arnd@arndb.de
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>