Simplify hwspinlock's Kconfig by making the global CONFIG_HWSPINLOCK
entry invisible; users will just select it when needed.

This also prepares the ground for adding hwspinlock support for other
platforms (the 'depends on ARCH_OMAP4' was rather hideous, and while
we're at it, a dedicated menu is added).
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>