drm/omap: add field for PLL type

DSS uses two types of PLLs, type A (DSI & Video) and type B (HDMI). The
two types behave slightly differently, but we don't have the type of the
PLL available anywhere for the driver.

This patch adds an enum for the PLL type and a field in the PLL's HW
data to store it.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>