arm64: dts: qcom: sc7280: Set SPI flash to 50 MHz for herobrine boards

sc7280-herobrine based boards are specced to be able to access their
SPI flash at 50 MHz with the drive strength of the pins set at 8. The
drive strength is already set to 8 in "sc7280-herobrine.dtsi", so
let's bump up the clock. The matching firmware change for this is at:

https://review.coreboot.org/c/coreboot/+/63948



NOTE: the firmware change isn't _required_ to make the kernel work at
50 MHz, it merely shows that the boards are known to work fine at 50
MHz.

ALSO NOTE: this doesn't update the "sc7280-chrome-common.dtsi" file
which is used by both herobrine boards and IDP. At the moment the IDP
boards aren't configuring a drive strength of 8 and it seems safer to
just leave them at the slower speed if they're already working.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Matthias Kaehlcke <mka@chromium.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20220505161425.1.Icf6f3796d2fa122b4c0566d9317b461bfbc24b7f@changeid