OMAP3+: Introduce generic logic for OMAP voltage controller

OMAP Voltage controller is used to generically talk to
PMICs on OMAP3,4,5 over I2C_SR. Instead of replicating code
in multiple SoC code, introduce a common voltage controller
logic which can be re-used from elsewhere.

With this change, we replace setup_sri2c with omap_vc_init which
has the same functionality, and replace the voltage scale
replication in do_scale_vcore and do_scale_tps62361 with
omap_vc_bypass_send_value. omap_vc_bypass_send_value can also
now be used with any configuration of PMIC.

NOTE: Voltage controller controlling I2C_SR is a write-only data
path, so no register read operation can be implemented.

Reported-by: Isabelle Gros <i-gros@ti.com>
Reported-by: Jerome Angeloni <j-angeloni@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>