ARM: davinci: ensure global variables are declared

Fix the following sparse warnings by declaring relevant
global variables.

  CHECK   arch/arm/mach-davinci/usb.c
arch/arm/mach-davinci/usb.c:134:12: warning: symbol 'da8xx_register_usb20' was not declared. Should it be static?
arch/arm/mach-davinci/usb.c:169:12: warning: symbol 'da8xx_register_usb11' was not declared. Should it be static?

  CHECK   arch/arm/mach-davinci/pm.c
arch/arm/mach-davinci/pm.c:155:12: warning: symbol 'davinci_pm_init' was not declared. Should it be static?

Signed-off-by: Sekhar Nori <nsekhar@ti.com>