ARM: ARTPEC-6: add select MFD_SYSCON to MACH_ARTPEC6

Since the ARTPEC-6 machine port already uses syscon,
MACH_ARTPEC6 should select MFD_SYSCON.

Signed-off-by: Niklas Cassel <niklas.cassel@axis.com>
Signed-off-by: Jesper Nilsson <jespern@axis.com>