fix(rmm): Report accurate breakpoint and watchpoint numbers

A Realm's ID_AA64DFR0_EL1 register is initialized with two breakpoints
and two watchpoints. These are the minimum values expected by the Arm
architecture, since ID_AA64DFR0_EL1 can only represent BP/WP counts
between 2 and 16. This limit is confirmed in DDI0487J.a D2.1.3 "About
debug exceptions", which states "The A-profile architecture provides
2-16 hardware breakpoints/watchpoints".

Write accurate breakpoints and watchpoints numbers in Feature Register
0, to avoid confusing a host that tries to create Realm parameters
compatible with both RMM and the architecture. Per the architecture,
values lower than two are invalid.

Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Change-Id: I34e2a08507eca164ade51f1c84a9b04e056474a8