intel: Turn `DEFAULT_RCBA` into a Kconfig symbol

Create `FIXED_RCBA_MMIO_BASE` and use it everywhere, except in cases
where a pointer cast would be necessary. Instances in Sandy Bridge MRC
code were left as-is intentionally, so as not to collide with another
cleanup patch train.

Tested with BUILD_TIMELESS=1, these boards remain identical:
- Asus P8Z77-V LX2
- Packard Bell MS2290

Change-Id: I642958fbd6f02dbf54812d6a75d6bc3087acc77a
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/50036
Reviewed-by: Nico Huber <nico.h@gmx.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
diff --git a/src/southbridge/intel/lynxpoint/bootblock.c b/src/southbridge/intel/lynxpoint/bootblock.c
index 802c58e..c063bfb 100644
--- a/src/southbridge/intel/lynxpoint/bootblock.c
+++ b/src/southbridge/intel/lynxpoint/bootblock.c
@@ -7,7 +7,7 @@
 
 static void map_rcba(void)
 {
-	pci_write_config32(PCH_LPC_DEV, RCBA, (uintptr_t)DEFAULT_RCBA | 1);
+	pci_write_config32(PCH_LPC_DEV, RCBA, CONFIG_FIXED_RCBA_MMIO_BASE | 1);
 }
 
 static void enable_port80_on_lpc(void)