x86: kconfig: Select ARCH_EARLY_INIT_R in the platform Kconfig

This is architecture-dependent early initialization hence should
be put in the platform Kconfig.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

board/advantech/som-db5800-som-6867/som-db5800-som-6867.c

@@ -17,8 +17,3 @@ int board_early_init_f(void)
 
 	return 0;
 }
-
-int arch_early_init_r(void)
-{
-	return 0;
-}