soc/intel/quark: Set CBMEM top from HW register

Properly obtain the top of memory address from the hardware registers
set by FSP.

TEST=Build and run on Galileo Gen2

Change-Id: I7681d32112408b8358b4dad67f8d69581c7dde2e
Signed-off-by: Lee Leahy <leroy.p.leahy@intel.com>
Reviewed-on: https://review.coreboot.org/15594
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martinroth@google.com>

src/soc/intel/quark/memmap.c

@@ -15,6 +15,8 @@
 
 #include <cbmem.h>
 #include <fsp/memmap.h>
+#include <soc/QuarkNcSocId.h>
+#include <soc/reg_access.h>
 
 size_t mmap_region_granularity(void)
 {
@@ -24,6 +26,16 @@ size_t mmap_region_granularity(void)
 
 void *cbmem_top(void)
 {
-	/* TODO: Get this dynamically*/
-	return (void *)0x0afd0000;
+	UINT32 top_of_memory;
+
+	/* Determine the TSEG base */
+	top_of_memory = reg_host_bridge_unit_read(QNC_MSG_FSBIC_REG_HSMMC);
+	top_of_memory &= SMM_START_MASK;
+	top_of_memory <<= 16;
+
+	/* Reserve 64 KiB for RMU firmware */
+	top_of_memory -= 0x10000;
+
+	/* Return the top of memory */
+	return (void *)top_of_memory;
 }