rpmsg: use proper format-specifier for printing dma_addr_t

The dma_addr_t types can be printed properly using the %pad
printk format-specifier, there is no need to resort to the
unsigned long long type-casting, to deal with different
possible type sizes.

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