Merged in masayuki2009/nuttx.nuttx/lc823450_smp_audio (pull request #547)

lc823450 smp audio

* arch/arm/src/lc823450: Remove a workaround in up_cpu_paused()

Signed-off-by: Masayuki Ishikawa <Masayuki.Ishikawa@jp.sony.com>

* arch/arm/src/lc823450: Introduce g_gpio_lock to improve write performance in SMP mode.

    NOTE: This is a tentative solution and should be replaced with more generic one.

Signed-off-by: Masayuki Ishikawa <Masayuki.Ishikawa@jp.sony.com>

* arch/arm/src/lc823450: Add a workaround in up_txready() to avoid data corruption.

Signed-off-by: Masayuki Ishikawa <Masayuki.Ishikawa@jp.sony.com>

* configs/lc823450-xgevk: Enable CONFIG_SMP for audio

Signed-off-by: Masayuki Ishikawa <Masayuki.Ishikawa@jp.sony.com>

* configs/lc823450-xgevk: Update README.txt regarding SMP audio

Signed-off-by: Masayuki Ishikawa <Masayuki.Ishikawa@jp.sony.com>

Approved-by: Gregory Nutt <gnutt@nuttx.org>