When KERNEL_PERF_EVENTS is enabled in OpenWrt, the RPI_AXIPERF symbol is
exposed. Add a build option for it to fix build failures with
KERNEL_PERF_EVENTS enabled.
Fixes: 20ea6adbf199 ("bcm27xx: add support for linux v5.15")
Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>