ARM: mach-mvebu: remove redundant DT parsing and validation

arm_dt_init_cpu_maps parses the device tree, validates and sets the
cpu_possible_mask appropriately. It is unnecessary to do another DT
parse to get the number of cpus, use num_possible_cpus instead.

Cc: Gregory Clement <gregory.clement@free-electrons.com>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Sudeep KarkadaNagesha <sudeep.karkadanagesha@arm.com>
Acked-by: Gregory Clement <gregory.clement@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>