ARMv7-R: fix to restore the Thumb flag in CPSR

Thumb flag in CPSR is not restored back when the context switch occurs
while executing thumb instruction.

Reported-by: Eunbong Song <eunb.song@samsung.com>
Signed-off-by: Byoungtae Cho <bt.cho@samsung.com>
Signed-off-by: Heesub Shin <heesub.shin@samsung.com>