arm64: sysreg: replace open-coded mrs_s/msr_s with {read,write}_sysreg_s

arm64: sysreg: replace open-coded mrs_s/msr_s with {read,write}_sysreg_s

Similar to our {read,write}_sysreg accessors for architected, named
system registers, this patch introduces {read,write}_sysreg_s variants
that can take arbitrary sys_reg output and therefore access IMPDEF
registers or registers that unsupported by binutils.

Reviewed-by: Mark Rutland <[email protected]>
Signed-off-by: Will Deacon <[email protected]>