qemu-arm: Remove need to specify flash banks

qemu-arm: Remove need to specify flash banks

Currently we hard-code the number and initial addresses of QEMU's flash
banks, even though our code is perfectly able to gather the same
information from the DTB provided by QEMU.
This is especially annoying, since we have two slightly different
U-Boot configurations ("bare-metal" vs. loaded via Arm Trusted
Firmware), which need to be selected at build time.

Drop the two hard coded alternatives, and use
CONFIG_SYS_MAX_FLASH_BANKS_DETECT instead, which relies on the DTB to
figure out the actual flash configuration at runtime.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>