ARM: virt: use PSR_N_BIT for detecting boot CPU mode mismatch

During boot, we detect whether or not all CPUs are brought up in the
same mode and signal this to the kernel using the N bit in the SPSR.

This patch tidies up the checking code to use the PSR_N_BIT macro,
rather than hardcoding the bit field and commenting it as such.

Signed-off-by: Will Deacon <will.deacon@arm.com>