clk: ti: k3-pll: Change DIV_CTRL programming to read-modify-write

clk: ti: k3-pll: Change DIV_CTRL programming to read-modify-write

There are three different divider values in the DIV_CTRL register
controlled by the k3-pll driver. Currently the ti_pll_clk_set_rate
function writes the entire register when programming plld, even though
plld only resides in the lower 6 bits.

Change the plld programming to read-modify-write to only affect the
relevant bits for plld and to preserve the other two divider values
present in the upper 16 bits, otherwise they will always get set to zero
when programming plld.

Fixes: 5a5f3984a2f7 ("clk: add support for TI K3 SoC PLL")
Signed-off-by: Dave Gerlach <d-gerlach@ti.com>