dts: arm: nxp: fix clock name for FTM nodes

Clock NXP_S32_FTMx_CLK reference the output of PCC_FTMx clocks, but
FTM has an internal clock mux to select the clock source of the
counter, which for ucans32k1sic board is set to system clock. Fix the
clock nodes to use the correct clock name. So far this was working
because both NXP_S32_FTMx_CLK and NXP_S32_CORE_CLK are configured to
the same frequency.

Fixes #74348

Signed-off-by: Manuel Argüelles <marguelles.dev@gmail.com>