vexpress64: Add BASER_FVP vexpress board variant

The BASER_FVP board variant is implemented on top of the BASE_FVP board
config (which, in turn, is based on the Juno Versatile Express board
config). They all share a similar memory map - for BASER_FVP the map is
inverted from the BASE_FVP
(https://developer.arm.com/documentation/100964/1114/Base-Platform/Base---memory/BaseR-Platform-memory-map

)

 * Create new TARGET_VEXPRESS64_BASER_FVP target, which uses the same
   board config as BASE_FVP and JUNO
 * Adapt vexpress_aemv8a.h header file to support BASER_FVP (and rename
   to vexpress_aemv8.h)
 * Enable config to switch to EL1 for the BASER_FVP
 * Create vexpress_aemv8r defconfig
 * Provide an MPU memory map for the BASER_FVP

For now, only single core boot is supported.

Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>
[trini: Add MAINTAINERS, move BOOTCOMMAND to defconfig]
Signed-off-by: Tom Rini <trini@konsulko.com>