The commit 503d0ea2
  ARM: OMAP4: hwmod data: Add aliases for McBSP fclk clocks

added a wrong "prcm_clk" alias for PRCM clock whereas the McBSP
driver and previous OMAPs are using "prcm_fck".

It thus lead to the following warning.

[   47.409729] omap-mcbsp: clks: could not clk_get() prcm_fck

Fix that by changing the opt_clk role to prcm_fck.
Reported-by: Misael Lopez Cruz <misael.lopez@ti.com>
Signed-off-by: Benoit Cousson <b-cousson@ti.com>
Cc: Peter Ujfalusi <peter.ujfalusi@ti.com>
Tested-by: Sebastien Guiriec <s-guiriec@ti.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>