mailbox: zynq: Switch to flexible array to simplify code

stable inclusion
from stable-v5.10.180
commit 67fb57f24737ffdbd443eb176fe3fc344c93f491
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I8FC2O

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=67fb57f24737ffdbd443eb176fe3fc344c93f491



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[ Upstream commit 043f85ce ]

Using flexible array is more straight forward. It
  - saves 1 pointer in the 'zynqmp_ipi_pdata' structure
  - saves an indirection when using this array
  - saves some LoC and avoids some always spurious pointer arithmetic

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>
Stable-dep-of: f72f805e ("mailbox: zynqmp: Fix counts of child nodes")
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: sanglipeng <sanglipeng1@jd.com>