fix: remove old-style declarations

TF-A wants to eventually enable -Wold-style-definition globally. Convert
the rare few instances where this is still the case.

Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
Change-Id: I9c450fc875cf097e6de2ed577ea3b085821c9f5e
diff --git a/plat/xilinx/common/ipi.c b/plat/xilinx/common/ipi.c
index 6438896..8fa7bc5 100644
--- a/plat/xilinx/common/ipi.c
+++ b/plat/xilinx/common/ipi.c
@@ -39,7 +39,7 @@
 #define IPI_BIT_MASK(I) (ipi_table[(I)].ipi_bit_mask)
 
 /* IPI configuration table */
-const static struct ipi_config *ipi_table;
+static const struct ipi_config *ipi_table;
 
 /* Total number of IPI */
 static uint32_t ipi_total;