ARM: vexpress: move HBI check to sysreg driver

The last reason for static memory mapping is the HBI (board
identification number) check early in the machine code.

Moving the check to the sysreg driver makes it possible to
completely remove the early mapping and init functions.

Signed-off-by: Pawel Moll <pawel.moll@arm.com>
Acked-by: Lee Jones <lee.jones@linaro.org>