cpumask: convert drivers/idle/i7300_idle.c to cpumask_var_t

Fairly simple transformation:
1) cpumask_t -> cpumask_var_t and alloc_cpumask_var/free_cpumask_var
   (which are a NOOP unless CONFIG_CPUMASK_OFFSTACK=y).
2) cpu_set -> cpumask_set_cpu
3) cpus_weight -> cpumask_weight
4) cpu_clear -> cpumask_clear_cpu

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
To: Andy Henroid <andrew.d.henroid@intel.com>