ep93xx: Make syscon access functions private to SoC

The syscon access functions are no longer used outside of the core
EP93xx code. Move their definitions into the SoC code.

Signed-off-by: Ryan Mallon <rmallon@gmail.com>
Reviewed-by: Mika Westerberg <mika.westerberg@iki.fi>
Acked-by: Hartley Sweeten <hsweeten@visionengravers.com>