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.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>