KVM: arm64: Extend reset_unknown() to handle mixed RES0/UNKNOWN registers

The reset_unknown() system register helper initialises a guest
register to a distinctive junk value on vcpu reset, to help expose
and debug deficient register initialisation within the guest.

Some registers such as the SVE control register ZCR_EL1 contain a
mixture of UNKNOWN fields and RES0 bits.  For these,
reset_unknown() does not work at present, since it sets all bits to
junk values instead of just the wanted bits.

There is no need to craft another special helper just for that,
since reset_unknown() almost does the appropriate thing anyway.
This patch takes advantage of the ununused val field in struct
sys_reg_desc to specify a mask of bits that should be initialised
to zero instead of junk.

All existing users of reset_unknown() do not (and should not)
define a value for val, so they will implicitly set it to zero,
resulting in all bits being made UNKNOWN by this function: thus,
this patch makes no functional change for currently defined
registers.

Future patches will make use of non-zero val.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>