refactor(qemu): use generic hold pen

Convert QEMU to use the common hold pen helpers.

This is not a functional fix -- QEMU uses HW_ASSISTED_COHERENCY=1
so caches are always coherent and the origina

refactor(qemu): use generic hold pen

Convert QEMU to use the common hold pen helpers.

This is not a functional fix -- QEMU uses HW_ASSISTED_COHERENCY=1
so caches are always coherent and the original 8-byte slots worked.
However, aligning with the common hold pen pattern improves
consistency and removes duplicated logic.

plat_hold_pen_init() is called once from plat_setup_psci_ops() to
write HOLD_STATE_WAIT into every slot and flush to main memory.
Then plat_hold_pen_signal() writes just the target slot with the
entrypoint. Secondaries use the plat_hold_pen_wait_and_jump macro
to poll and branch directly to the entrypoint stored in their slot,
eliminating the mailbox fetch.

Change-Id: I5ca1ca5fb8708024ce8a04fa80403473dcd1e257
Signed-off-by: Nicolas Pitre <npitre@baylibre.com>

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