arch/x86/postcar_loader: Don't add postcar to stage cache

In romstage, CBMEM and TSEG are not yet cached, so reading/writing to
the stage cache is slow. This means there isn't really any benefit to
adding postcar to the stage cache:
 - During boot, adding it to the stage cache adds a few ms of boot time,
   since writing to uncached memory is slow.
 - During S3 resume, reading from stage cache takes roughly the same
   time as reading from SPI, so we get little or no savings anyway.

E.g. on nivviks, this change saves 6 ms of boot time and 2 ms of S3
resume time.

Adding postcar to stage cache:
Boot:
  rmodule_stage_load()     - 4 ms
  stage_cache_add()        - 6 ms
S3 resume:
  stage_cache_load_stage() - 6 ms

Not adding postcar to stage cache:
Boot:
  rmodule_stage_load()     - 4 ms
S3 resume:
  rmodule_stage_load()     - 4 ms

Savings:
Boot time      - 6 ms
S3 resume time - 2 ms

------------------------------------
Savings on other devices:

                          nivviks anahera kindred treeya
rmodule_stage_load()          4.0     1.4     0.8    2.8
stage_cache_add()             6.0     2.8     1.5    2.0
stage_cache_load_stage()      6.0     2.8     1.5    2.0

Boot time savings             6.0     2.8     1.5    2.0
S3 resume time savings        2.0     1.4     0.7   -0.8

Note AMD platforms from zork onwards don't use postcar due to PSP
verstage.

BUG=b:247940538
TEST=On nivviks, boot and check S3 resume still works. Add logging to
check postcar is added to stage cache when POSTCAR_IN_STAGE_CACHE is
selected, and not added otherwise.

Change-Id: I3614c0874a6c71d13606b0b782ea445692d88bb1
Signed-off-by: Reka Norman <rekanorman@chromium.org>
---
M src/arch/x86/Kconfig
M src/arch/x86/postcar_loader.c
2 files changed, 70 insertions(+), 2 deletions(-)