ARM: mmp: allow platform devices with modular USB

The USB host drivers need platform data to be defined on
pxa168 and pxa910, but the conditionals used in the devices.c
file only work if the drivers are built-in. This patch
fixes the definition by changing the #ifdef to #if IS_ENABLED(),
which works both for built-in and modular Kconfig symbols.

I found one specific problem using 'randconfig' builds, but
for consistency, this patch uses IS_ENABLED() for all Kconfig
symbols in these three files.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Haojian Zhuang <haojian.zhuang@gmail.com>