am335x: add some missing CM_CLKMODE_DPLL_SSC macros

add missing CM_CLKMODE_DPLL_SSC_ACK_MASK,
CM_CLKMODE_DPLL_SSC_DOWNSPREAD_MASK and
CM_CLKMODE_DPLL_SSC_TYPE_MASK
defines. Used for enabling spread spectrum.

Signed-off-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Tom Rini <trini@konsulko.com>