OMAP4: AuxCoreBoot registers only accessible in secure mode

The AuxCoreBoot0 and AuxCoreBoot1 can be only accessed in secure
mode. Replace the current code with secure monitor API's to access/modify
these registers.

Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>