ARM: ux500: Fix PMU IRQ regression

Commit 2b05f6ae ("ARM: ux500: remove PMU IRQ bouncer")
deleted some code to bounce and work around the weird PMU
IRQs in the DB8500 ASIC, but did a semantic mistake:
since the auxdata was now unused, the call to
of_platform_populate() was removed, but this does not
work: the default platform population will only kick in
if .init_machine() is assigned NULL, and since the U8540
was still using the callback that was not the case.

Fix this by reinstating the call to of_platform_populate(),
but pass NULL as auxdata.

Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Fixes: 2b05f6ae ("ARM: ux500: remove PMU IRQ bouncer")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>