fix: remove non-VHE builds platform directories

Following non-VHE builds deprecation [1], remove leftover platform
directories that are no longer built.

Merge earlier rpi platform support into rpi4.

[1] https://review.trustedfirmware.org/c/hafnium/project/reference/+/18792



Signed-off-by: Olivier Deprez <olivier.deprez@arm.com>
Change-Id: Ieb96e54d724cd633bed08787da51d1652d9c7741