arch/x86: Move ehci_dbg_info outside _car_relocatable_data

As code already used CBMEM hooks to switch from CAR to CBMEM
it was never necessary to have the structure declared inside
_car_relocatable_data.

Switch to use car_[get|set]_ptr is mostly for consistency, but
should also enable use of usbdebug with FSP1.0 romstage.

Change-Id: I636251085d84e52a71a1d5d27d795bb94a07422d
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/35288
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>

src/arch/x86/car.ld

@@ -59,13 +59,13 @@
 
 	TIMESTAMP(., 0x200)
 
-	_car_relocatable_data_start = .;
-
 	_car_ehci_dbg_info_start = .;
 	/* Reserve sizeof(struct ehci_dbg_info). */
         . += 80;
         _car_ehci_dbg_info_end = .;
 
+	_car_relocatable_data_start = .;
+
 	/* _car_global_start and _car_global_end provide symbols to per-stage
 	 * variables that are not shared like the timestamp and the pre-ram
 	 * cbmem console. This is useful for clearing this area on a per-stage