ARM: OMAP1: USB: fix ocpi_enable compile problem on non-1610 builds

ARM: OMAP1: USB: fix ocpi_enable compile problem on non-1610 builds

Janusz Krzysztofik reported the following build break on OMAP1 builds that
don't include CONFIG_ARCH_OMAP16XX:

  LD      .tmp_vmlinux1
arch/arm/mach-omap1/built-in.o: In function `omap1_usb_init':
lcd_dma.c:(.init.text+0x1420): undefined reference to `ocpi_enable'
make: *** [.tmp_vmlinux1] Error 1

This was caused by commit d3645d39ad0ed9f09535065676ea0ba114f93cdf
("ARM: OMAP1: OHCI: use platform_data fn ptr to enable OCPI bus").
Fix by declaring an empty ocpi_enable() on non-16XX builds, which
should work until the OCPI code is moved out to drivers/.

Cc: Janusz Krzysztofik <jmkrzyszt@gmail.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>