rpmsg: rpc: use %pa for printing phys_addr_t

The phys_addr_t types can be printed properly using the %pa
printk format-specifier, there is no need to type-cast the
corresponding variables to a different type and print them
accordingly.

Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Suman Anna <s-anna@ti.com>