mailbox: zynq: Switch to flexible array to simplify code

[ 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>