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In preparation for SMCCC v1.1 support, save x4 to x29 unconditionally.
Previously we expected callers coming from AArch64 mode to preserve
x8-x17.  This is no longer the case with SMCCC v1.1 as AArch64 callers
only need to save x0-x3.

Change-Id: Ie62d620776533969ff4a02c635422f1b9208be9c
Signed-off-by: Dimitris Papastamos <dimitris.papastamos@arm.com>