ARM: AM43xx: fix dpll init in bypass mode

On AM43xx, if a PLL is in bypass at kernel init, the code in
omap2_get_dpll_rate() will not realize this and will try to calculate
the clock rate using the multiplier and the divider, resulting in
errors.

omap2_init_dpll_parent() has similar issue.

Add the missing soc_is_am43xx() check to make the code work on AM43xx.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Sathya Prakash M R <sathyap@ti.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>