Some STM32MP Linux drivers, which could be ported to barebox as part of
improved second stage barebox support, are using hardware spinlocks to
coordinate access between the three processors. While barebox only
uses one of the two Cortex-A cores, spin locks may still be necessary
because of firmware loaded unto the Cortex-M.

To ease porting these drivers, without having to import the hw spinlock
driver now, add stubs for the functions.

The stubs will we be reused when CONFIG_HWSPINLOCK is introduced,
but is disabled in config. The API is oriented after the implementation
in U-Boot.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>