src/cpu,soc/amd/common/block/cpu: Add preload_microcode

This will enable preloading the microcode. By preloading the
file, into cbfs_cache we reduce boot time. 

BUG=b:179699789
TEST=Boot guybrush with CL chain and see microcode preloading and a
reduction of 1 ms.
| 112 - started reading uCode                         | 1.041     | 1.204     Δ(  0.16,    0.01%) |
| 113 - finished reading uCode                        | 1.365     | 0.011     Δ( -1.35,   -0.10%) |

Signed-off-by: Raul E Rangel <rrangel@chromium.org>
Change-Id: If0c634c692c97769e71acd1175fc464dc592c356
Reviewed-on: https://review.coreboot.org/c/coreboot/+/58963
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Karthik Ramasubramanian <kramasub@google.com>

src/include/cpu/amd/microcode.h

@@ -2,5 +2,6 @@
 #define CPU_AMD_MICROCODE_H
 
 void amd_update_microcode_from_cbfs(void);
+void preload_microcode(void);
 
 #endif /* CPU_AMD_MICROCODE_H */