Fix RISC-V configMTIMECMP_BASE_ADDRESS (64-bit) stored in 32-bit int

[vishwamartur]

Related to #189

Update configMTIMECMP_BASE_ADDRESS to be stored in a 64-bit integer.

• Change the type of ullMachineTimerCompareRegisterBase to uint64_t in portable/GCC/RISC-V/port.c.
• Change the type of ullMachineTimerCompareRegisterBase to uint64_t in portable/IAR/RISC-V/port.c.
• Update the initialization of ullMachineTimerCompareRegisterBase to use configMTIMECMP_BASE_ADDRESS in both files.