Make the syscon-poweroff driver change only the shutdown bits on the
SNVS_LP control register.

This change fixes an issue with the RTC wakeup alarm, that was
previously disabled during power off.
Signed-off-by: NGuy Shapiro <guy.shapiro@mobi-wize.com>
Reviewed-by: NMarcin Niestroj <m.niestroj@grinn-global.com>
Reviewed-by: NFabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: NShawn Guo <shawnguo@kernel.org>