lib/prog_loaders: Add payload_preload

This method will allow the SoC code to start loading the payload before
it is required.

BUG=b:177909625
TEST=Boot guybrush and see read/decompress drop by 23 ms.

Signed-off-by: Raul E Rangel <rrangel@chromium.org>
Change-Id: Ifa8f30a0f4f931ece803c2e8e022e4d33d3fe581
Reviewed-on: https://review.coreboot.org/c/coreboot/+/56051
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin Roth <martinroth@google.com>

src/include/program_loading.h

@@ -154,6 +154,13 @@ void run_ramstage(void);
 
 int payload_arch_usable_ram_quirk(uint64_t start, uint64_t size);
 
+/*
+ * Asynchronously preloads the payload.
+ *
+ * This should be called early on to allow the payload to load before
+ * `payload_load` is called.
+ */
+void payload_preload(void);
 /* Load payload into memory in preparation to run. */
 void payload_load(void);