Fix the musb initialization sequence on AM335x.

According to http://e2e.ti.com/support/arm/sitara_arm/f/791/t/210729 the
USB reset pulse has an undocumented duration of 200ns and during this
period the module must not be acessed.

We wait for 100us to take into account for some imprecision of the early
DELAY() loop.

This fixes the eventual 'External Non-Linefetch Abort (S)' that happens at
boot while resetting the musb subsystem.

While here, enable the USB subsystem clock before the first access.

Discussed with: ian, adrian
MFC after: 2 weeks