x86: timer: use a timer base of 0

On x86 platforms the timer is reset to 0 when the SoC is reset. Having
this as the timer base is useful since it provides an indication of how
long it takes before U-Boot is running.

When U-Boot sets the timer base to something else, time is lost and we
no-longer have an accurate account of the time since reset. This
particularly affects bootstage.

Change the default to not read the timer base, leaving it at 0. Add an
option for when U-Boot is the secondary bootloader.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/arch/x86/cpu/slimbootloader/Kconfig b/arch/x86/cpu/slimbootloader/Kconfig
index 3ea4c99..58a9ca0 100644
--- a/arch/x86/cpu/slimbootloader/Kconfig
+++ b/arch/x86/cpu/slimbootloader/Kconfig
@@ -17,3 +17,4 @@
 	imply USB_EHCI_HCD
 	imply USB_XHCI_HCD
 	imply E1000
+	imply X86_TSC_READ_BASE