uarts: 32/64 cleanup

We had lots of casts that caused warnings when compiling on RISCV.
Clean them up.

Change-Id: I46fcb33147ad6bf75e49ebfdfa05990e8c7ae4eb
Signed-off-by: Ronald G. Minnich <rminnich@gmail.com>
Reviewed-on: http://review.coreboot.org/7066
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
diff --git a/src/drivers/uart/uart8250io.c b/src/drivers/uart/uart8250io.c
index f4f4d01..60a8d86 100644
--- a/src/drivers/uart/uart8250io.c
+++ b/src/drivers/uart/uart8250io.c
@@ -105,7 +105,7 @@
 
 static const unsigned bases[] = { 0x3f8, 0x2f8, 0x3e8, 0x2e8 };
 
-unsigned int uart_platform_base(int idx)
+uintptr_t uart_platform_base(int idx)
 {
 	if (idx < ARRAY_SIZE(bases))
 		return bases[idx];