x86: Add a config for the systemagent PCIEX regions size

Add a way to specify the required size for this region. This is used when
generating ACPI tables.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/x86/cpu/apollolake/Kconfig b/arch/x86/cpu/apollolake/Kconfig
index 99d4e10..37d6289 100644
--- a/arch/x86/cpu/apollolake/Kconfig
+++ b/arch/x86/cpu/apollolake/Kconfig
@@ -13,6 +13,7 @@
 	select TPL_X86_TSC_TIMER_NATIVE
 	select SPL_PCH_SUPPORT
 	select TPL_PCH_SUPPORT
+	select PCIEX_LENGTH_256MB
 	select PCH_SUPPORT
 	select P2SB
 	select SMP_AP_WORK