[ARM] generic: Reduce max VCPU count for ARMv5, ARMv6, and ARMv7

For processors without virtualization extensions our VAPOOL size
cannot be increased beyond 12 MB because Linux ARM has only 12 MB
of empty/unused/reserved virtual address space where Xvisor can run.

Having max VCPU count of 64 for ARMv5, ARMv6, and ARMv7 means TTBL
pool will consume 2 MB which is too much for 12 MB of VAPOOL.

This patch reduces max VCPU count to 48 in generic-v5-defconfig,
generic-v6-defconfig, and generic-v7-defconfig so that TTBL pool
burden on VAPOOL reduces.

Signed-off-by: Anup Patel <anup@brainfault.org>