fix(st-uart): allow 64 bit compilation

Change a %x in %zx to print a size_t variable.

Signed-off-by: Yann Gautier <yann.gautier@foss.st.com>
Change-Id: I27ac3739f4a2ec3b33c34d2257fa858cbd1aae6a
diff --git a/plat/st/common/stm32cubeprogrammer_uart.c b/plat/st/common/stm32cubeprogrammer_uart.c
index e4a5338..0916099 100644
--- a/plat/st/common/stm32cubeprogrammer_uart.c
+++ b/plat/st/common/stm32cubeprogrammer_uart.c
@@ -409,7 +409,7 @@
 	handle.addr = (uint8_t *)buffer;
 	handle.len = length;
 
-	INFO("UART: read phase %u at 0x%lx size 0x%x\n",
+	INFO("UART: read phase %u at 0x%lx size 0x%zx\n",
 	     id, buffer, length);
 	while (!start_done) {
 		ret = uart_receive_command(&command);