drivers: soc: xilinx: fix firmware driver Kconfig dependency

drivers: soc: xilinx: fix firmware driver Kconfig dependency

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: c8e5c25c65f3 ("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>