ARM: pxa: mark unused eseries code as __maybe_unused

Two variables in eseries.c are used on multiple platforms,
but are not referenced when those are all disabled:

eseries.c:60:31: warning: 'e7xx_gpio_vbus' defined but not used [-Wunused-variable]
eseries.c:129:20: warning: 'eseries_register_clks' defined but not used [-Wunused-function]

Marking them __maybe_unused is the nicest way to ensure
that we never get the warning or end up with missing symbols
if we get it wrong.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>