soc: arm: microchip_mec: remove re-defined Kconfig help

The help for MCHP_ECIA_XEC is defined in
drivers/interrupt_controller/Kconfig.xec. defconfig files must not
re-define prompts or help, just modify the default value.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>