[ARM] locomo: avoid unnecessary cascaded keyboard IRQ

It is not necessary and is over-complicated for IRQ_LOCOMO_KEY to
be a cascaded IRQ of IRQ_LOCOMO_KEY_BASE. Removed and introduced
locomokbd_{open,close} for masking/unmasking of the keyboard IRQ.

Signed-off-by: Eric Miao <eric.y.miao@gmail.com>