boards/tms570ls31x-usb-kit: Set LOOPSPERMSEC=0 to avoid build errors

This board does not have a calibrated LOOPSPERMSEC value despite needing
one in order for correct operation. This commits sets the value to 0 so
that CI builds may pass, but users who build this board will see a
warning indicate the steps for calibrating a new value.

Signed-off-by: Matteo Golin <matteo.golin@gmail.com>

Author: linguini1

boards/arm/tms570/tms570ls31x-usb-kit/configs/nsh/defconfig

@@ -17,6 +17,7 @@ CONFIG_ARCH_INTERRUPTSTACK=2048
 CONFIG_ARCH_IRQBUTTONS=y
 CONFIG_ARCH_LOWVECTORS=y
 CONFIG_ARCH_STACKDUMP=y
+CONFIG_BOARD_LOOPSPERMSEC=0
 CONFIG_BUILTIN=y
 CONFIG_ELF=y
 CONFIG_FAT_LCNAMES=y