hwrng: reorder OMAP TRNG driver code

The newly added omap4 support in the driver was added without
consideration for building older configs. When building omap1_defconfig,
it resulted in:

drivers/char/hw_random/omap-rng.c:190:12: warning: 'omap4_rng_init' defined but not used [-Wunused-function]
drivers/char/hw_random/omap-rng.c:215:13: warning: 'omap4_rng_cleanup' defined but not used [-Wunused-function]
drivers/char/hw_random/omap-rng.c:251:20: warning: 'omap4_rng_irq' defined but not used [-Wunused-function]

Move the code around so it is grouped with its operations struct, which
for the omap4 case means also under the #ifdef CONFIG_OF, where it needs
to be.

Signed-off-by: Olof Johansson <olof@lixom.net>
Cc: Lokesh Vutla <lokeshvutla@ti.com>