include: arch: riscv: drop __soc_get_irq() declaration
include: arch: riscv: drop __soc_get_irq() declaration

There is no __soc_get_irq() definition in the Zephyr source
code tree.

Signed-off-by: Antony Pavlov <e7e2558ecf173848a6564d3e4b1af569555a907d@gmail.com>

zephyrproject-rtos/zephyr,finikorg/zephyr,nashif/zephyr,galak/zephyr,finikorg/zephyr,Vudentz/zephyr,Vudentz/zephyr,galak/zephyr,nashif/zephyr,finikorg/zephyr,finikorg/zephyr,Vudentz/zephyr,Vudentz/zephyr,Vudentz/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,nashif/zephyr,nashif/zephyr,Vudentz/zephyr