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/silk_defconfig b/configs/silk_defconfig
index 9ff9e23..5005e16 100644
--- a/configs/silk_defconfig
+++ b/configs/silk_defconfig
@@ -105,3 +105,4 @@
 CONFIG_USB_EHCI_HCD=y
 CONFIG_USB_EHCI_PCI=y
 CONFIG_USB_STORAGE=y
+CONFIG_SYS_TIMER_COUNTS_DOWN=y