ARM: exynos: Clear global variable on init error path

For most of Exynos SoCs, Power Management Unit (PMU) address space is
mapped into global variable 'pmu_base_addr' very early when initializing
PMU interrupt controller.  A lot of other machine code depends on it so
when doing iounmap() on this address, clear the global as well to avoid
usage of invalid value (pointing to unmapped memory region).

Properly mapped PMU address space is a requirement for all other machine
code so this fix is purely theoretical.  Boot will fail immediately in
many other places after following this error path.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>