counter: microchip-tcb-capture: Add capture extensions for registers RA/RB

counter: microchip-tcb-capture: Add capture extensions for registers RA/RB

TCB hardware is capable of capturing the timer value to registers RA and
RB. Add these registers as capture extensions.

Signed-off-by: Bence Csókás <csokas.bence@prolan.hu>
Link: https://lore.kernel.org/r/20250306134441.582819-3-csokas.bence@prolan.hu
Signed-off-by: William Breathitt Gray <wbg@kernel.org>