ARM: EXYNOS: fix pm code to check for cortex A9 rather than the SoC

We have an soc check to ensure that the scu and certain A9 specific
registers are not accessed on Exynos5250 (which is A15 based).
Rather than adding another soc specific check for 5420 let us test
for the Cortex A9 primary part number.

This resolves the below crash seen on exynos5420 during core switching
after the CPUIdle consolidation series was merged.

[  155.975589] [<c0013174>] (scu_enable) from [<c001b0dc>] (exynos_cpu_pm_notifier+0x80/0xc4)
[  155.983833] [<c001b0dc>] (exynos_cpu_pm_notifier) from [<c003c1b0>] (notifier_call_chain+0x44/0x84)
[  155.992851] [<c003c1b0>] (notifier_call_chain) from [<c007a49c>] (cpu_pm_notify+0x20/0x3c)
[  156.001089] [<c007a49c>] (cpu_pm_notify) from [<c007a564>] (cpu_pm_exit+0x20/0x38)
[  156.008635] [<c007a564>] (cpu_pm_exit) from [<c0019e98>] (bL_switcher_thread+0x298/0x40c)
[  156.016788] [<c0019e98>] (bL_switcher_thread) from [<c003842c>] (kthread+0xcc/0xe8)
[  156.024426] [<c003842c>] (kthread) from [<c000e438>] (ret_from_fork+0x14/0x3c)
[  156.031621] Code: ea017fec c0530a00 c052e3f8 c0012dcc (e5903000

Signed-off-by: Abhilash Kesavan <a.kesavan@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>