ARM: mvebu: use dt_fixup to provide fallback for enable-method

We need to maintain backwards compatibility with device trees that don't
define an enable method. At the same time we want the device tree to be
able to specify an enable-method and have it stick.

Previously by having smp assigned in the DT_MACHINE definition this
would be picked up by setup_arch() and override whatever
arm_dt_init_cpu_maps() had configured. Now we move the initial
assignment of default smp_ops to a dt_fixup and let
arm_dt_init_cpu_maps() override that if the device tree defines an
enable-method.

[olof@lixom.net: Wrap set_smp_ops() in CONFIG_SMP.]
Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Tested-by: Gregory CLEMENT <gregory.clement@bootlin.com> (on AX3)
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>