zephyr: add CONFIG_ISL9241_SWITCHING_FREQ to allowlist
zephyr: add CONFIG_ISL9241_SWITCHING_FREQ to allowlist

Add the CONFIG_ISL9241_SWITCHING_FREQ to the allowlist to avoid a
compile error on Chromium EC builds.

A follow on CL converts the CONFIG_ISL9241_SWITCHING_FREQ to a
devicetree property.

BUG=none
BRANCH=none
TEST=make buildall
TEST=zmake testall

Signed-off-by: Keith Short <f6d578f97033c25f1206cb57d52f358974c82954@chromium.org>
Change-Id: Ic16f3a7747dd2a32f25f9c8acfd8f50310fa1910
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2748446
Reviewed-by: Fabio Baltieri <1c55e39c05d9ab42261288cf330256b2af6ba179@google.com>
Reviewed-by: Yuval Peress <50dc11c009bd4d7e9d9d15a383c1743ce5ee82ef@chromium.org>
Commit-Queue: Yuval Peress <50dc11c009bd4d7e9d9d15a383c1743ce5ee82ef@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec