tests: drivers: spi_loopback: increase latency limit for nrf54h

The nrf54h20 needs a slightly increased latency limit as the latency
is around 650us at 8MHz, which is right above the current limit of
648us (CONFIG_SPI_IDEAL_TRANSFER_DURATION_SCALING=12).

Increase CONFIG_SPI_IDEAL_TRANSFER_DURATION_SCALING to 15 to match
cpuppr_xip and match the observed latency.

Signed-off-by: Bjarki Arge Andreasen <bjarki.andreasen@nordicsemi.no>
diff --git a/tests/drivers/spi/spi_loopback/boards/nrf54h20dk_nrf54h20_cpuapp.conf b/tests/drivers/spi/spi_loopback/boards/nrf54h20dk_nrf54h20_cpuapp.conf
index ad922ab..7259243 100644
--- a/tests/drivers/spi/spi_loopback/boards/nrf54h20dk_nrf54h20_cpuapp.conf
+++ b/tests/drivers/spi/spi_loopback/boards/nrf54h20dk_nrf54h20_cpuapp.conf
@@ -1 +1 @@
-CONFIG_SPI_IDEAL_TRANSFER_DURATION_SCALING=12
+CONFIG_SPI_IDEAL_TRANSFER_DURATION_SCALING=15
diff --git a/tests/drivers/spi/spi_loopback/boards/nrf54h20dk_nrf54h20_cpurad.conf b/tests/drivers/spi/spi_loopback/boards/nrf54h20dk_nrf54h20_cpurad.conf
index ad922ab..7259243 100644
--- a/tests/drivers/spi/spi_loopback/boards/nrf54h20dk_nrf54h20_cpurad.conf
+++ b/tests/drivers/spi/spi_loopback/boards/nrf54h20dk_nrf54h20_cpurad.conf
@@ -1 +1 @@
-CONFIG_SPI_IDEAL_TRANSFER_DURATION_SCALING=12
+CONFIG_SPI_IDEAL_TRANSFER_DURATION_SCALING=15