Convert CONFIG_SYS_TIMER_COUNTS_DOWN to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_TIMER_COUNTS_DOWN

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/porter_defconfig b/configs/porter_defconfig
index 83cc54a..ca56008 100644
--- a/configs/porter_defconfig
+++ b/configs/porter_defconfig
@@ -103,3 +103,4 @@
 CONFIG_USB_EHCI_HCD=y
 CONFIG_USB_EHCI_PCI=y
 CONFIG_USB_STORAGE=y
+CONFIG_SYS_TIMER_COUNTS_DOWN=y