target/arm: update the cortex-a15 MIDR to latest rev

target/arm: update the cortex-a15 MIDR to latest rev

QEMU doesn't model micro-architectural details which includes most
chip errata. The ARM_ERRATA_798181 work around in the Linux
kernel (see erratum_a15_798181_init) currently detects QEMU's
cortex-a15 as broken and triggers additional expensive TLB flushes as
a result.

Change the MIDR to report what the latest silicon would (r4p0). We
explicitly set the IMPDEF revidr bits to 0 because we don't need to
set anything other than the silicon revision to indicate these flushes
are not needed. This cuts about 5s from my Debian kernel boot with the
latest 6.0rc1 kernel (29s->24s).

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Anders Roxell <anders.roxell@linaro.org>
Message-id: 20221010153225.506394-1-alex.bennee@linaro.org
Cc: Arnd Bergmann <arnd@linaro.org>
Cc: Anders Roxell <anders.roxell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Anders Roxell <anders.roxell@linaro.org>
Message-Id: <20220906172257.2776521-1-alex.bennee@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>