ARM: davinci: psc: fix incorrect mask

There are 5 LSB bits defined in PDSTAT and the code
currently uses a mask of 1 bit to check the status.

Use a proper mask per the hardware specification.
While at it, use a #define for the mask to improve
readability.

Reviewed-by: Sergei Shtylyov <sshtylyov@mvista.com>
Signed-off-by: Murali Karicheri <m-karicheri2@ti.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>