ARM: shmobile: Use pm-rmobile on sh7372 and r8a7740 only

The functions in pm-rmobile.c are only used on sh7372 and
r8a7740, so adjust the Makefile to only link in that file
in case those SoCs are selected.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>