[ARM] pxa/sharpsl_pm: merge pxa-specific code into generic one

As pxa now is the only user of sharpsl_pm we can drop several startup
functions into generic code thus dropping several global functions.

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Signed-off-by: Eric Miao <eric.miao@marvell.com>