ep93xx: Move EP93XX_SYSCON defines to SoC private header

The EP93XX_SYSCON defines are now no longer needed outside of the
EP93xx SoC core code, so they can be moved to a private header.

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