ARM: firmware: Introduce suspend and resume operations

This patch extends the firmware_ops structure with two new callbacks:
.suspend() and .resume(). The former is intended to ask the firmware to
save all its volatile state and suspend the system, without returning
back to the kernel in between. The latter is to be called early by
very low level platform suspend code after waking up to restore low
level hardware state, which can't be restored in non-secure mode.

While at it, outdated version of the structure is removed from the
documentation and replaced with a reference to the header file.

Signed-off-by: Tomasz Figa <t.figa@samsung.com>
Acked-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>