[ARM] ohci-pxa27x: introduce pxa27x_clear_otgph()

Direct access to pxa27x specific register PSSR in a generic ohci driver
is no good, introduce pxa27x_clear_otgph() and move the implementation
into processor specific code.

Signed-off-by: Eric Miao <eric.miao@marvell.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>