Reduce the use of size_t.

Many uses have more specific types that should be used instead. In
particular, size_t was used for register sized types so uintreg_t is
introduced to be more explicit and portable.

Change-Id: Ib8038bdd9cd164e6d9b1f42d10d4b1654a665997
diff --git a/inc/hf/boot_params.h b/inc/hf/boot_params.h
index f7eb324..7b486d5 100644
--- a/inc/hf/boot_params.h
+++ b/inc/hf/boot_params.h
@@ -18,6 +18,8 @@
 
 #include <stdbool.h>
 
+#include "hf/arch/cpu.h"
+
 #include "hf/mm.h"
 
 #define MAX_MEM_RANGES 20
@@ -32,7 +34,7 @@
 	size_t mem_ranges_count;
 	paddr_t initrd_begin;
 	paddr_t initrd_end;
-	size_t kernel_arg;
+	uintreg_t kernel_arg;
 };
 
 struct boot_params_update {