drivers/cpuidle: make cpuidle-ux500.c explicitly non-modular

The Kconfig currently controlling compilation of this code is:

cpuidle/Kconfig.arm:config ARM_U8500_CPUIDLE
cpuidle/Kconfig.arm:    bool "Cpu Idle Driver for the ST-E u8500 processors"

...meaning that it currently is not being built as a module by anyone.

Lets remove the couple traces of modularity so that when reading the
driver there is no doubt it is builtin-only.

Since module_platform_driver() uses the same init level priority as
builtin_platform_driver() the init ordering remains unchanged with
this commit.

Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>