clk: at91: cleanup PMC header file for PCR register fields

Add _MASK and _OFFSET values and cleanup register fields layout.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>