[ARM] pxa: introduce reset_status and clear_reset_status for driver's usage

Due to the problem of reset status bits being handled by different
registers between pxa2xx and pxa3xx, introduce a global reset_status
variable, initialized by SoC-specific code and later being used by
other drivers.

And also introduce clear_reset_status(), which is used to clear the
corresponding status bits. Pass RESET_STATUS_ALL to clear all bits.

Signed-off-by: Eric Miao <eric.miao@marvell.com>