clk: imx5: Fix i.MX50 mainbus clock registers

i.MX50 does not have a periph_apm clock. Instead, the main bus clock
(a.k.a. periph_clk) comes directly from a MUX between pll1_sw, pll2_sw,
pll3_sw, and lp_apm.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>