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/northbridge/intel/i945/early_init.c b/src/northbridge/intel/i945/early_init.c
index dea4f9b..4564ff4 100644
--- a/src/northbridge/intel/i945/early_init.c
+++ b/src/northbridge/intel/i945/early_init.c
@@ -730,7 +730,7 @@
 	reg32 |= (1 << 0);
 	DMIBAR32(DMILE1D) = reg32;
 
-	DMIBAR32(DMILE1A) = (uintptr_t)DEFAULT_RCBA;
+	DMIBAR32(DMILE1A) = CONFIG_FIXED_RCBA_MMIO_BASE;
 
 	DMIBAR32(DMILE2D) |= (1 << 16) | (1 << 0);