drivers: rpmsg: rpm-smd: define readq_relaxed for ARM variants

ARM cannot natively read 64-bit address. As such the readq_ variants
are not defined for this architecture. Add a 32-bit equivalent to read 2
sequential 4-byte memory into a u64 value. Let's keep them locally
defined for now, since they are not native to the ARM architecture.

Change-Id: I9d00d0a9cd185e658d7b860cac2f59c8438e1b30
Signed-off-by: Raghavendra Kakarla <rkakarla@codeaurora.org>