arm,kgdb: fix GDB_MAX_REGS no longer used

According to commit 22eeef4b



    kgdb,arm: Individual register get/set for arm

It's now replaced by DBG_MAX_REG_NUM.

Signed-off-by: Eric Miao <eric.y.miao@gmail.com>
Signed-off-by: Jason Wessel <jason.wessel@windriver.com>