arm64/sysreg: Fix checks for incomplete sysreg definitions

The checks for incomplete sysreg definitions were checking if the
next_bit was greater than 0, which is incorrect and missed occasions
where bit 0 hasn't been defined for a sysreg. The reason is that
next_bit is -1 when all bits have been processed (LSB - 1).

Change the checks to use >= 0, instead. Also, set next_bit in Mapping
to -1 instead of 0 to match these new checks.

There are no changes to the generated sysreg definitons as part of
this change, and conveniently no definitions lack definitions for bit
0.

Signed-off-by: Sascha Bischoff <sascha.bischoff@arm.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>