imxrt: hold EVKB Ethernet PHY in reset while tracing, 50 MHz trace pin

Fresh bring-up pass on mimxrt1170_evkb: holding the 100M RTL8201 in reset
(ENET_RST_B = GPIO_LPSR_04) stops its RMII lines driving against the
shared trace pads and doubles the clean trace-pin rate to 50 MHz
(100 MHz CSTRACE root; 133 MHz root is marginal, stock 132 corrupts).
Validated 3x 8 s TinyUSB captures at 11.46M fetches. D1-D3 remain silent
in every configuration - the welded R1882-R1884 are electrically open;
reflow is the remaining step to width 4. Board notes gain JP4 (must be
shorted for an external probe on J58).