boot/edk2: fix the build for arm sgi575

The edk2 package can be configured for platform Arm Sgi575 but this
does not build correctly:

Usage: build.exe [options] [all|fds|genc|genmake|clean|cleanall|cleanlib|modules|libraries|run]

build.exe: error: option -a: invalid choice: '-b' (choose from 'IA32', 'X64', 'EBC', 'ARM', 'AARCH64', 'RISCV64')
make[1]: *** [package/pkg-generic.mk:293: /home/thomas/buildroot/buildroot/output/build/edk2-edk2-stable202102/.stamp_built] Error 2
make: *** [Makefile:84: _all] Error 2

Add the necessary definitions to fix the build.

Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>
Cc: Dick Olsson <hi@senzilla.io>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>