soc/amd/common/block/spi: fix setting SPI_USE_SPI100 in SPI100_ENABLE

Use a read modify write sequence when setting the SPI_USE_SPI100 bit in
the SPI100_ENABLE register. This avoids clearing other bits in the
register which might cause instabilities of the SPI interface. The
reference code for both Picasso and Cezanne also only sets the
SPI_USE_SPI100 bit and doesn't zero out the other bits.

TEST=Verified that Mandolin still boots. It didn't show any signs of
possibly related instabilities before though, so this test doesn't say
much.

Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Change-Id: I71c2ec1729d5cb4cdff6444b637af29caaa6f1c0
Reviewed-on: https://review.coreboot.org/c/coreboot/+/59932
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Marshall Dawson <marshalldawson3rd@gmail.com>

0 files changed, 0 insertions, 0 deletions