zephyr: Tidy up PLATFORM_EC_USBC_SS_MUX
zephyr: Tidy up PLATFORM_EC_USBC_SS_MUX

From what I can tell, there is no need for PLATFORM_EC_USB_MUX since
the only mux is the SuperSpeed one.

Drop PLATFORM_EC_USB_MUX and move the runtime-config one up into the
same area.

BUG=b:183296099
BRANCH=none
TEST=Build lazor on zephyr; no obvious changes when run

Signed-off-by: Simon Glass <c00b0378376498bd9cd974c388df8854c0131d27@chromium.org>
Change-Id: I4ac673ea92c43a87bbfcfe578a86f603ff9b9a00
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2822396
Reviewed-by: Yuval Peress <50dc11c009bd4d7e9d9d15a383c1743ce5ee82ef@chromium.org>
Reviewed-by: Jack Rosenthal <d3f605bef1867f59845d4ce6e4f83b8dc9e4e0ae@chromium.org>
Reviewed-by: Wai-Hong Tam <04b587fdf5845741a0c5a8b9cd59ca72d73ef8fc@google.com>
Commit-Queue: Wai-Hong Tam <04b587fdf5845741a0c5a8b9cd59ca72d73ef8fc@google.com>

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