hwrng: omap - Add OMAP4 TRNG support

Add support for OMAP4 version of TRNG module
that is present on OMAP4, AM33xx and OMAP5 SoCs.

The modules have several differences including register
offsets, output size, triggering rng and how configuring
FROs. To handle these differences, a platform_data structure
is defined and contains routine pointers, register offsets. OMAP2
specific routines are prefixed with 'omap2_' and OMAP4
specific routines are prefixed with 'omap4_'.

Note: Few Hard coded values are from the TI AM33xx SDK.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>