soc: litex: Add function for reading 64bit CSRs
soc: litex: Add function for reading 64bit CSRs

This adds a helper function for reading LiteX 64-bit CSRs
to be used by LiteX drivers.

Signed-off-by: Jakub Cebulski <5f9cb5b66c2861005f2d497129df698fa734bf72@internships.antmicro.com>
Signed-off-by: Mateusz Holenko <1a79f421828d0048009fccfd7504972981cac7dd@antmicro.com>

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