soc/amd/common/block/lpc/spi_dma: Yield after completing transaction

There is no telling when the next udelay will be, so explicitly call
`thread_yield()` after completing a transaction. This will allow any
pending transactions to immediately start.

BUG=b:179699789
TEST=Verify new transaction is enqueued right after another.

Signed-off-by: Raul E Rangel <rrangel@chromium.org>
Change-Id: I9c1272bde46c3e0c15305b76c2ea7a6dde5ed0b0
Reviewed-on: https://review.coreboot.org/c/coreboot/+/56321
Reviewed-by: Karthik Ramasubramanian <kramasub@google.com>
Reviewed-by: Furquan Shaikh <furquan@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/soc/amd/common/block/lpc/spi_dma.c

@@ -193,6 +193,9 @@ static ssize_t spi_dma_readat_dma(const struct region_device *rd, void *destinat
 	printk(BIOS_SPEW, "%s: end: dest: %p, source: %#zx, remaining: %zu\n",
 	       __func__, destination, source, transaction.remaining);
 
+	/* Allow queued up transaction to continue */
+	thread_yield();
+
 	if (transaction.remaining)
 		return -1;