ARC: MWDT: workaround paddr_t defined in both Zephyr and toolchain

MWDT provides paddr_t type and it conflicts witn Zephyr definition:
- Zephyr defines paddr_t as a uintptr_t
- MWDT defines paddr_t as a unsigned long
This causes extra warnings. However we can safely define
paddr_t as a unsigned long for the case when MWDT toolchain is used as
they are both unsighned, have same size and aligning.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Evgeniy Paltsev <PaltsevEvgeniy@gmail.com>