ARM: dts: qcom: Set default_gen_speed as gen3 for sdx65

Set pcie default_gen_speed as gen3 for sdx65.
Disable device initiated BW scaling in sdxlemur-cnss.dtsi,
then host can set pcie linkspeed as gen3, ignore gen2 request
from hsp firmware event:MHI_ER_BW_SCALE_ELEMENT_TYPE.

Change-Id: Ib0ecfdf68098decb5448ee692b63ef8ff52eacdc
CRs-Fixed: 3044666