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Normally, SGI6 & SGI7 is used by non-secure world, these
two SGIs should not be reserved for secure interrupt purpose.
On i.MX8M platform, SGI8 is used for secure group0 IPI for
DDR DVFS, So update the code to reserve SGI8 for secure world.

Change-Id: Ib1ed9786e0a79bb729b120a0d4d791d13b6f048a
Signed-off-by: Jacky Bai <ping.bai@nxp.com>