arch: arm: pm_s2ram: fix compatibility with ARMv6-M again

The original 'arch_pm_s2ram_resume' implementation saved lr on the stack
using 'push {lr}' and restored it using 'pop {lr}'. However, the Thumb-1
'pop' does not support lr as a target register, so this code would not
compile for ARMv6-M or ARMv8-M Baseline. r0 was added to these push/pop
later in 2590c48d.

In 474d4c32, arch_pm_s2ram* functions were
modified to no longer use the stack, which incidentally "fixed" this issue.
b4fb5d38 reverted this commit and brought
back 'pop {r0, lr}' as-is, without taking compatibility into account.

Modify the sequence to use "pop {r0, pc}" which is supported on all
ARM M-profile implementations (v6/v7/v8 Baseline/v8 Mainline), and
add comments to (hopefully) prevent this issue from re-appearing.

Signed-off-by: Mathieu Choplain <mathieu.choplain@st.com>