CBMEM intel: Define get_top_of_ram() once per chipset

Only have one definition of get_top_of_ram() function and compile
it using __SIMPLE_DEVICE__ for both romstage and ramstage.

Implemented like this on intel/northbridge/gm45 already.
This also adds get_top_of_ram() to i945 ramstage.

Change-Id: Ia82cf6e47a4c929223ea3d8f233d606e6f5bf2f1
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: http://review.coreboot.org/3993
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@google.com>

src/northbridge/intel/sandybridge/raminit.c

@@ -302,10 +302,3 @@ void sdram_initialize(struct pei_data *pei_data)
 	if (pei_data->boot_mode != 2)
 		save_mrc_data(pei_data);
 }
-
-unsigned long get_top_of_ram(void)
-{
-	/* Base of TSEG is top of usable DRAM */
-	u32 tom = pci_read_config32(PCI_DEV(0,0,0), TSEG);
-	return (unsigned long) tom;
-}