OMAP2 clock: APLL code shouldn't rely on static clocks in its local namespace

Similar to the previous patch, the APLL code relied on the presence of the
static struct clks in its own namespace.  The APLL code didn't use them for
validation, however - it adjusted its own internal state depending on
the struct clk * that called it.  Now that static struct clks are
leaving the clock24xx.c namespace, use a more durable method: split the
omap2_clk_fixed_enable() function into omap2_clk_apll96_enable() and
omap2_clk_apll54_enable().  They still share a disable function.

Signed-off-by: Paul Walmsley <paul@pwsan.com>