usb: ehci: add weak-aliased function for PORTSC

There is at least one non-EHCI compliant controller (i.e. Faraday EHCI)
not only leave RESERVED and CONFIGFLAG registers un-implemented
but also has their address spaces removed.

As an result, the PORTSC register of Faraday EHCI always
starts from 0x30 instead of 0x44 in standard EHCI.

So that we'll need a weak-aliased function for abstraction.

Signed-off-by: Kuo-Jung Su <dantesu@faraday-tech.com>
CC: Marek Vasut <marex@denx.de>