arm64: initialize all of CNTHCTL_EL2

In __init_el2_timers we initialize CNTHCTL_EL2.{EL1PCEN,EL1PCTEN} with a
RMW sequence, leaving all other bits UNKNOWN.

In general, we should initialize all bits in a register rather than
using an RMW sequence, since most bits are UNKNOWN out of reset, and as
new bits are added to the reigster their reset value might not result in
expected behaviour.

In the case of CNTHCTL_EL2, FEAT_ECV added a number of new control bits
in previously RES0 bits, which reset to UNKNOWN values, and may cause
issues for EL1 and EL0:

* CNTHCTL_EL2.ECV enables the CNTPOFF_EL2 offset (which itself resets to
  an UNKNOWN value) at EL0 and EL1. Since the offset could reset to
  distinct values across CPUs, when the control bit resets to 1 this
  could break timekeeping generally.

* CNTHCTL_EL2.{EL1TVT,EL1TVCT} trap EL0 and EL1 accesses to the EL1
  virtual timer/counter registers to EL2. When reset to 1, this could
  cause unexpected traps to EL2.

Initializing these bits to zero avoids these problems, and all other
bits in CNTHCTL_EL2 other than EL1PCEN and EL1PCTEN can safely be reset
to zero.

This patch ensures we initialize CNTHCTL_EL2 accordingly, only setting
EL1PCEN and EL1PCTEN, and setting all other bits to zero.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Oliver Upton <oupton@google.com>
Cc: Will Deacon <will@kernel.org>
Reviewed-by: Oliver Upton <oupton@google.com>
Acked-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210818161535.52786-1-mark.rutland@arm.com
Signed-off-by: Will Deacon <will@kernel.org>

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