usb: musb: Remove platform context save/restore API

For OMAP3 and OMAP4 for offmode and retention support, musb
sysconfig is configured to force idle and standby with ENABLE_FORCE bit
of OTG_FORCESTNDBY set.
And on wakeup configure to no ilde/standby with resetting the ENABLE_FORCE
bit. There is not need to save and restore of this register anymore
so removed omap2430_save_context/omap2430_restore_context functions.
and also removed otg_forcestandby member of musb_context_registers
structure

Signed-off-by: Hema HK <hemahk@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>