irqchip: mips-gic: Print warning if inherited GIC base is used

If the physical address of the GIC resource cannot be read from device
tree, then the code falls back to reading it from the gcr_gic_base
register. Hopefully this has been set to a sane value by the bootloader
or some platform code, but is defined by the hardware manual to have
"undefined" reset state. Using it as the address at which the GIC will
be mapped into physical memory space can therefore be risky if it has
not been initialised, since it may result in the GIC being mapped to an
effectively random address anywhere in physical memory, where it might
conflict with peripherals or RAM and lead to weird crashes.

Since a "sane value" is very platform specific because it is particular
to the platform's memory map, it is difficult to test for. At the very
least, a warning message should be printed in the case that we trust the
inherited value.

Reported-by: Amit Kama <amit.kama@satixfy.com>
Signed-off-by: Matt Redfearn <matt.redfearn@mips.com>
Reviewed-by: Paul Burton <paul.burton@mips.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>