ARM: ux500: Include the PRCMU's Secure Registers in DB8500's DT

Currently we only include the PRCMU's primary registers when
referencing the register count in the 'reg' property. This patch
expands that count to include the secure registers also.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>