libpayload: refactor fetching cbmem pointers

There's a recurring pattern of reading cbtable entries that point into
cbmem entries. Move that pattern into its own function.

Coccinelle patch used for this:
  @@
  identifier T, T2;
  expression TARGET;
  @@
  -struct cb_cbmem_tab *const T2 = (struct cb_cbmem_tab *)T;
  -TARGET = phys_to_virt(T2->cbmem_tab);
  +TARGET = get_cbmem_ptr(T);

Change-Id: I7bd4a7ad8baeeaebf0fa7d4b4de6dbc719bc781f
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/35756
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Furquan Shaikh <furquan@google.com>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

payloads/libpayload/include/coreboot_tables.h

@@ -395,4 +395,6 @@ static inline const char *cb_mb_part_string(const struct cb_mainboard *cbm)
 	(void *)(((u8 *) (_rec)) + sizeof(*(_rec)) \
 		+ (sizeof((_rec)->map[0]) * (_idx)))
 
+/* Helper functions */
+void *get_cbmem_ptr(unsigned char *ptr);
 #endif