arm64: Documentation: Fix broken table in generated HTML

cpu-feature-registers.rst is missing a new line before a couple
of tables listing the visible fields, causing broken tables in
the HTML documentation generated by "make htmldocs". Fix this
by adding the missing new line.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20200707143152.154541-1-suzuki.poulose@arm.com


Signed-off-by: Will Deacon <will@kernel.org>