mailbox: zynq: Switch to flexible array to simplify code

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>