arm/dts: 8974pro: Set deemphasis value to override default setting

driver is currently setting deemphasis value to 22 for all 8974
variants. This value needs to be set to 26 for USB3 complaince
tests passing for 8974 pro. Hence set deemphasis value to 26 in
device tree file.

CRs-Fixed: 536912
Change-Id: Ib14c4a213a435bf8c89810ce7a741d27950d1851
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>