ARM: mvebu: add __initconst specifiers on DT_MACHINE_START dt_compat tables

As suggested by Stephen Boyd, this commit adds the __initconst
specifier to the dt_compat table declarations used by the
DT_MACHINE_START structures in mach-mvebu land.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>