clk: mmp2: separate LCDC peripheral clk form the display clock

These are in fact two clocks, they shouldn't be exposed as one. One is
required for accessing LCD controller registers (peripheral clock), while
other (AXI clock) can be optionally used as a pixel clock source for the
panel.

LCDC can alternatively use different clocks than the Display 1 AXI clock
for generating the pixel clock: the second AXI clock (fixed in this
commit too), the HDMI PLL, or the AXI bus clock.

They should really be controlled independently.

Link: https://lists.freedesktop.org/archives/dri-devel/2019-January/203975.html


Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>